70 INTRODUCTION TO SEXUAL PHYSIOLOGY 



spleen and the urinary and generative organs (except the epi- 

 thelium of the bladder and urethra). 



The endoderm gives rise to the epithelium of the alimentary 

 canal (except at its extreme ends) and its glandular appendages 

 (liver, pancreas, etc.), the epithelium of the respiratory cavity, 

 Eustachian tubes and tympanum, the epithelium of the thyroid 

 vesicles and of the nests of the thymus, and the epithelium of 

 the bladder and urethra (except near the external opening). 



Attachment of the Embryo 



In certain of the lower mammals the developing embryo 

 remains free in the uterine cavity for a somewhat prolonged 

 period. Thus in the cow it is still unattached on the seventeenth 

 day of pregnancy, and its first connection with the uterine mucosa 

 is by means of the yolk sac from which villous processes grow 

 outwards and anchor the embryo and its membranes to the wall 

 of the uterus. The area of attachment is also an absorbing area, 

 nourishment being transmitted from the mother through the villi 

 of the yolk sac. With the growth of the chorion, however, the 

 contents of the yolk sac are gradually absorbed into the embryo, 

 and, as we have seen, for the human embryo all that eventually 

 remains of the yolk sac is what is represented by the alimentary 

 canal. The attachment by the yolk sac is replaced by that 

 through the chorion at about the fourth or fifth week in the cow, 

 and at about the sixth or seventh week in the mare. In most 

 marsupials (kangaroos, etc.) the yolk sac represents the sole 

 mechanism of embryonic attachment, and after its release the 

 young is born in an exceedingly immature condition, and forth- 

 with transferred to the marsupium or pouch, where it is nourished 

 through the teat. 



In all the higher mammals, however (the placental mammals), 

 the yolk sac functions for only a short time as a mechanism of 

 attachment, and in man it probably does not function at all. 

 We have seen that the young segmenting human ovum, even as 

 early as the seventh day, is provided with an outer trophoblastic 

 covering, two cell layers deep. The outer of these gives rise 

 to the syncytium, which appears to have the power of destroy- 

 ing the uterine mucosa, probably by means of a ferment. By 

 this means the segmenting ovum bores or eats its way into the 



