GREGORY PINCUS 
occurs cyclically and the periodic production of eggs is 
halted only by pregnancy or, in animals, by anoestrus. The 
ovulated egg is normally fertilized in the Fallopian tube, travels 
through the lower portion of the tube for some days, enters the 
uterus, and eventually implants in the endometrium (lining of 
the uterus) where its growth into a foetus takes place. I should 
like to consider experimental data concerned with: (1) The 
control of ovulation in animals and some applications to the 
human and (2) some aspects of the physiology of ovum develop- 
ment preceding implantation. 
CONTROL OF OVULATION 
A general concept of ovulation-controlling mechanisms may 
be framed as follows. ‘The hypothalamus (an area at the base 
of the brain) produces a substance, known as a neurohumour, 
which enters the anterior lobe of the pituitary gland and acts 
upon it to stimulate the secretion into the blood of a hormone 
(gonadotropin) generally called luteinizing hormone (LH). 
It is LH which enters the ovary from the blood to stimulate the 
pre-ovulatory swelling and final rupture of the follicle. The 
corpus luteum formed from the ruptured follicle produces large 
amounts of (steroidal) progestational hormone exemplified by 
progesterone. Progesterone in turn acts upon the hypothalamus 
to inhibit the production of the LH-stimulating neurohumour. 
Thus, since progesterone is secreted in large amount during 
pregnancy, ovulation cannot occur during that critical period. 
There are a number of variations to this general scheme but it 
is basic to the ovulation process in all mammals. 
It is clear from the foregoing that there are four loci essential 
to ovulation and that each of these is open to experimental 
attack. These loci are: (1) The hypothalamus, essentially a 
nervous structure; (2) the anterior pituitary, essentially a 
secretory gland; (3) the general blood circulation as a carrier 
of gonadotropin (LH); and (4) the ovarian follicle. Experi- 
mental studies of ovulation control have largely centred on 
control of hypothalamic activity leading to ovulation. This 
is because it was known that progesterone and also a number 
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