264 EMBRYOLOGY 



mones. Great caution should be exercised therefore, in extrapolating results 

 from animals to humans. 



Vitamin E 



In addition to the hormones a vitamin is necessary in the early embryonic 

 development of mammals. In the absence of vitamin E, ovulation and im- 

 plantation of the blastocyst in the walls of the uterus may be normal but the 

 embryo is resorbed in early stages of development. This resorption is pre- 

 sumably caused by a failure in the circulatory system of the embryo, leading 

 to death. In the egg of the chick, lack of vitamin E results in a breakdown of 

 the blood system at about the fourth day of incubation and the embryo dies. 

 In humans lack of vitamin E may possibly be associated with habitual abor- 

 tion, in which the early fetus dies and is cast out of the uterus. On the other 

 hand, a low progesterone content may also produce the same effect. Therefore 

 a combination of vitamin E from wheat germ oil and progesterone is some- 

 times injected into women who experience difficulty in carrying a fetus. 



It is rather interesting that vitamin E is also necessary for reproduction 

 in the males of some mammals. Lack of vitamin E results, first, in a loss of 

 fertilizing power of the sperm. With continued lack of vitamin E, sperm 

 are no longer produced in the testis. This vitamin acts rather specifically on 

 sperm production, since neither the accessory glands nor the hormone pro- 

 duction in the male are altered. 



In retrospect, we see that in following this special case of the mammal 

 we have been carried very far from the basic problems of growth. All the 

 hormones we have considered plus vitamin E have nothing to do with the 

 differentiation or true development processes in the mammalian embryo. 

 They are merely concerned with establishing a relationship between the de- 

 veloping embryo and the uterine wall such that substances may diffuse to 

 and from the mother and the embryo. As we shall see in the next chapter, 

 the actual mechanics of development and the principles governing differ- 

 entiation in the mammal are very similar to those which we have found in 

 the frog and chick studies. The introduction of the placenta as a transfer 

 mechanism between parent and offspring is, however, accompanied by the 

 development of an elaborate hormone control. The general result of such 

 a mechanism is a greater frequency in survival of the offspring. 



