Hans Spemann

Chapter 1 · A Noose of Baby Hair

Making instruments fine enough to ask the question

1869 – 1903 · Stuttgart · Heidelberg · Würzburg

Hans Spemann was born in Stuttgart in June 1869, the eldest son of a publisher and bookseller. He began in medicine, moved to zoology, and took his doctorate at Würzburg under Theodor Boveri — who was demonstrating, with sea-urchin eggs, that chromosomes are individual and that each is necessary for normal development.

Spemann's subject was the amphibian embryo: newts and salamanders, whose eggs are large, tough enough to handle, and develop outside the body where they can be watched.

The problem was tools. He wanted to interfere with an embryo — to constrict it, divide it, move pieces about — at a scale of a fraction of a millimetre, and no instrument existed. So he made them: glass needles drawn out over a flame, pipettes pulled to a hair's width, tiny glass bridges.

And, most famously, a ligature. He needed something fine, strong and flexible enough to tie round an embryo and tighten gradually without cutting it. He used a strand of hair from his infant son's head — baby hair being exceptionally fine and soft.

With it, around 1902, he tied a constriction round a newt embryo at the two-cell stage and pulled it tight enough to divide the cells completely.

Both halves developed into complete, normal larvae. Two whole animals from one embryo — identical twins, made deliberately.

Why this matters

Spemann's questions were limited entirely by his instruments, so he made them himself — including a ligature of his own baby's hair.

You have the hair noose and the two complete larvae. What would you ask?

Ask Spemann

• “Why a hair from your own son's head?”
• “What does getting two whole larvae from one embryo prove?”
• “Why work on newts rather than something else?”
• “What did Boveri teach you?”
• “Does the instrument really decide what can be known?”

Chapter 2 · When Does a Cell Decide What It Will Be?

Splitting at different stages, and the answer that emerged

1903 – 1918 · Würzburg · Rostock · Berlin

The two-cell result was the beginning of a question. If splitting an embryo at the two-cell stage gives two whole animals, when does that stop being true?

Wilhelm Roux had argued for a *mosaic* view: each cell of an early embryo already contains its instructions for a particular part, so an embryo is a mosaic of pieces already assigned. Roux had killed one cell of a two-cell frog embryo with a hot needle and got a half-embryo, which supported it.

Spemann's constrictions gave a different answer, and the difference turned out to be about *where* he tied the knot.

When he divided a newt embryo along one plane, both halves made complete larvae. When he divided it along another plane, one half made a complete larva and the other made only an unstructured mass of belly tissue.

So the cells are not all equivalent, and they are not all pre-assigned either. Something is present in one part of the egg and not another — some region carrying the information that organises a body. If your knot puts it in both halves, you get two animals. If it goes entirely into one half, the other cannot build anything.

That localised the question precisely. There is a *place* in the embryo that matters. The next step was to find out what it does when you move it.

He also showed, by transplanting tissue between embryos at different stages, that a piece taken early adapts to its new surroundings while the same piece taken later builds what it would have built at home. Fate is progressively fixed, not fixed from the start.

Why this matters

Spemann showed that developmental fate is progressively determined rather than assigned from the beginning, which settled a central dispute in embryology.

You have the knot that matters and the region it contains. What is your question?

Ask Spemann

• “Why does it matter where you tie the knot?”
• “What was wrong with Roux's mosaic theory?”
• “How do you show that fate becomes fixed over time?”
• “What happens to the half that cannot build anything?”
• “How do you interrupt development at the right moment?”

Chapter 3 · Hilde Mangold's Hands

The organiser, 1921 to 1924

1921 – 1924 · Freiburg

Hilde Pröscholdt came to Freiburg as Spemann's doctoral student in 1920. The experiment he set her was extremely difficult and she performed it.

The procedure: take a small piece of tissue from the *upper lip of the blastopore* — the region where cells fold inward at the start of gastrulation — from one newt embryo, and graft it into the flank of a second embryo, on the side that would normally become belly.

To tell host tissue from graft, she used two species with differently pigmented eggs, so that under the microscope the transplanted cells could be identified by colour.

The result: the host embryo grew a *second, complete body axis* — a second neural tube, notochord, somites, and in some cases a nearly complete second larva joined to the first.

And the pigment showed the crucial thing. The second body was built mostly out of *host* cells. The graft did not become a second embryo. It *instructed the surrounding tissue* to become one. Belly cells that would have made belly were persuaded to make a back.

Spemann named the region the *organiser*, and the process *induction*: one group of cells changing the fate of another.

Of hundreds of attempted operations, a handful survived to give the result. The paper appeared in 1924 under both names.

Hilde — by then married to Otto Mangold, another of Spemann's assistants — died in September 1924, aged twenty-six, when a petrol heater exploded in her kitchen. Her son was an infant. She did not live to see the work recognised.

“The organiser does not build the second embryo; it persuades the host to build it.”

— Attributed to Hans Spemann, on the 1924 organiser experiment

Why this matters

The organiser does not build the second embryo — it persuades the host to build it, which is why induction rather than transplantation is the discovery.

You have the graft, the pigment and the second body. What would you ask?

Ask Spemann

• “Why does it matter that the second body is made of host cells?”
• “What made the operation so difficult?”
• “How many attempts failed for each one that worked?”
• “What is the organiser actually sending out?”
• “Tell me about Hilde Mangold.”

Chapter 4 · The Prize She Did Not Share

1935, and a question about credit

1935 · Stockholm · Freiburg

The 1935 Nobel Prize in Physiology or Medicine was awarded to Hans Spemann alone, for the discovery of the organiser effect in embryonic development.

Hilde Mangold had been dead for eleven years, and the Nobel Prize is not awarded posthumously — so the committee could not have named her. That is the straightforward part.

The harder question is what would have happened had she lived. Historians of science have debated it and there is no settled answer. She was a doctoral student, and the convention of the period gave the professor the credit for work done in his laboratory as a matter of course. She was a woman in German academic science in the 1920s, a position of very limited prospects. And she performed the operation, developed the technique to the point where it worked, and is first author on the 1924 paper.

What is clear is that Spemann conceived the experiment and interpreted it, and that Mangold executed it. Whether that is one discovery or two is exactly the kind of question scientific credit handles badly.

There is a further difficulty about Spemann. He remained a German professor through the rise of the Nazi regime, retiring from his chair in 1937. He was not a party member and did not publicly endorse the regime; nor did he oppose it, and colleagues who were dismissed on racial grounds — including Jewish scientists in German biology — did not find him a defender. That is a common position among German academics of the period, and it is not a defence.

He died at Freiburg in September 1941, aged seventy-two.

Why this matters

The organiser paper's first author performed the experiment and is absent from the prize, which makes it a standing case in how scientific credit is assigned.

You have the prize awarded to one name. What is your question?

Ask Spemann

• “Would Hilde Mangold have shared the prize if she had lived?”
• “How much of the discovery was the idea and how much the hands?”
• “What were the prospects for a woman in German science then?”
• “What did you do when colleagues were dismissed under the regime?”
• “How should a professor credit a student now?”

Chapter 5 · The Fantastical Experiment

A sentence written in 1938 that produced a sheep in 1996

1938 – 1996 · Freiburg · Roslin

In his 1938 book *Embryonic Development and Induction*, Spemann described an experiment he could not perform and called it *fantastical*.

The proposal: take a cell from an embryo that has already begun to specialise. Remove its nucleus. Take an egg and remove *its* nucleus. Put the first nucleus into the emptied egg. And see whether a whole animal can be built from it.

The question behind it is precise and fundamental. When a cell specialises — becomes a skin cell, a muscle cell — does it *lose* the genetic information for everything else? Or does it keep the whole set and simply switch most of it off? If the second, then a differentiated nucleus put into an egg should be able to build a complete animal, because it still has all the instructions.

Spemann had no means of attempting it. Nobody could remove a nucleus from a cell and place it in an egg with the tools of 1938.

Briggs and King did it with frog embryonic cells in 1952. John Gurdon did it with cells from a tadpole's gut in the early 1960s and got swimming tadpoles — showing that even a specialised cell keeps the whole genome.

And on 5 July 1996, at the Roslin Institute near Edinburgh, a lamb was born from the nucleus of a mammary-gland cell taken from a six-year-old ewe. Dolly settled it for mammals.

Gurdon shared the Nobel Prize in 2012, fifty years after his frogs, for work that answered a question written down by a man who could only state it.

Why this matters

Spemann's question — does a specialised cell keep the whole genome? — was stated precisely enough in 1938 that others could answer it decades later.

You have the experiment described and not attempted. What would you ask?

Ask Spemann

• “Why call it a fantastical experiment?”
• “Does a specialised cell lose the genes it is not using?”
• “Is describing an experiment you cannot do a real contribution?”
• “What would you have made of Dolly?”
• “How do you state a question so that somebody later can test it?”

What Spemann changed

Induction is the foundation of developmental biology, and the organiser is still taught under Spemann's name. He established that developmental fate is progressively determined rather than assigned from the start. And his 'fantastical experiment' of 1938 was performed on amphibians in the 1950s and 1960s and on a mammal in 1996 — so the cloning of animals begins as a sentence in a book by a man who had no way to attempt it.

A debate that continues

Hilde Mangold performed the decisive organiser transplant and is first author on the 1924 paper, and whether she would have shared the 1935 prize had she lived is genuinely unresolved. Spemann's conduct as a German professor through the Nazi period was passive rather than resistant.

Keep exploring — ask Spemann

• “What other experiment would you describe for somebody else to do?”
• “How does one group of cells tell another what to become?”
• “What should Hilde Mangold be remembered for?”

Related lives

• Ian Wilmut — Whose Lamb Had No Father
• Rosalind Franklin — The Dark Lady of DNA
• Santiago Ramón y Cajal — Father of Modern Neuroscience
• Gregor Mendel — Father of Genetics

Related themes

Development and differentiation · Cells and specialisation · Credit in science