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If you are preparing for NEET, you meet de Graaf's name in every menstrual-cycle question: the Graafian follicle. Few students know that he worked without a microscope, made a famous mistake, and died at 32. This post covers what he found, how his work maps onto your NCERT chapter, and a short calculation that puts the numbers in perspective.
Who was Regnier de Graaf?
De Graaf was born on 30 July 1641 and died on 17 August 1673, dates given in the title of a 2014 profile in *Molecular Reproduction and Development*. According to a profile in *Archives of Pathology & Laboratory Medicine* (2000), he studied at Utrecht and Leiden, where he worked under Franciscus Sylvius. In 1664 he published a study of pancreatic juice, collected from a live dog through a tube placed in the pancreatic duct.
He took his doctorate in medicine at Angers in France in July 1665. Because he was a Catholic, a post at the Protestant University of Leiden was closed to him, so he set up a busy practice in Delft. In 1668 he published a book on the male reproductive organs, showing that the testis is made of fine tubules.
What did his 1672 book show?
*De mulierum organis generationi inservientibus*, printed at Leiden in 1672, is scanned on the Internet Archive. Its subtitle claims that humans and other animals that bear live young come from an egg, just as egg-laying animals do.
The *Archives of Pathology* profile summarises what he did. He gave the first thorough description of the female gonad, then called the "female testicle", and proposed that it be called the ovary. He did not claim to discover the fluid-filled vesicles, since Vesalius and Falloppio had seen them; he was the first to describe how the ovary changes before and after mating, and to describe the corpus luteum. He understood that the fallopian tube carries the egg to the uterus and knew of tubal pregnancies.
He also made a revealing error. Without a microscope, he thought each whole follicle was the egg. Tracing rabbit embryos in the oviduct to the third day after mating, he was puzzled that they were smaller than the follicles. The mammalian egg itself was found by Karl Ernst von Baer in 1827.
His last years were bitter. Jan Swammerdam accused him of taking credit for work by Swammerdam and Johannes van Horne, a dispute covered in our post on Jan Swammerdam.
How did de Graaf help Leeuwenhoek?
In a letter from Delft dated 28 April 1673, de Graaf told the Royal Society about a local man who had made microscopes better than any before. The Society printed it as "A specimen of some observations made by a microscope, contrived by M. Leewenhoeck" in the *Philosophical Transactions*. That introduction began the career covered in our post on Leeuwenhoek and microbes. De Graaf died less than four months later.
What does NCERT say about the Graafian follicle?
Your NCERT Class 12 Biology chapter "Human Reproduction" describes the sequence. At puberty only 60,000–80,000 primary follicles are left in each ovary. A primary follicle becomes secondary, then tertiary (with a fluid-filled antrum), then the mature Graafian follicle. LH and FSH peak around the 14th day of a cycle that averages 28/29 days; the LH surge ruptures the Graafian follicle and releases the secondary oocyte (ovulation). The remains form the corpus luteum, which secretes progesterone.
Worked example: how many follicles ever ovulate?
Use NCERT's figures, plus one sample assumption: 28-day cycles running for 35 years.
Follicles at puberty (both ovaries): 2 × 60,000 = 120,000 to 2 × 80,000 = 160,000.
Number of cycles: 35 × 365 ÷ 28 = 456 (rounded down).
One ovum per cycle: about 456 ovulations.
Fraction that ever ovulate: 456 ÷ 160,000 = 0.29% to 456 ÷ 120,000 = 0.38%.
So well under 1% of follicles present at puberty ever release an oocyte.
Common mistakes in NEET questions
Order of phases. Menstrual, follicular, ovulatory, luteal.
Which hormone triggers ovulation. The LH surge, not FSH alone.
Corpus luteum timing. It forms after ovulation, from the ruptured follicle.
Calling it the ovum. NCERT says the follicle releases the secondary oocyte (ovum).
If the hormone sequence will not stick, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can read each step on its own. Our human reproduction guide for NEET has practice questions.
What can a student take from de Graaf?
De Graaf compared ovaries across many animals, credited earlier observers, and tracked changes over time instead of describing one specimen. His mistake came from the limits of his tools, not careless thinking, and he still pointed the field in the right direction.
Practice routine for this week:
Draw and label a Graafian follicle from memory, then check against NCERT Figure 2.7.
Sketch the hormone graph for a 28-day cycle and mark day 14.
Write three lines on what de Graaf got right and one on what he got wrong.
Start with the drawing; labelling from memory shows quickly what you have not yet learned.
