CHAP, ii.] PREGNANCY AND BIRTH. 381 



development around the villi of epithelial structures of a secretory 

 character; in ruminant animals collections of such cells form what 

 is called ' uterine milk.' It is in these cells belonging to the border 

 line between mother and infant, whether they are of maternal or of 

 embryonic origin, that the glycogen, which is so often present in the 

 placenta, is placed, and the presence of this substance may be taken 

 as a token of the metabolic activity of these cells. 



At times, in the human subject, in what is called " extra-uterine 

 gestation," the embryo undergoes considerable development, not 

 in the cavity of the uterus, but outside it, generally in the 

 Fallopian tube. In such cases the nutrition of the embryo is 

 effected by a vascular connection between the chorionic villi and 

 the mucous membrane of the tube, and even apparently with the 

 adjoining peritoneum. This shews that the uterus is not essential 

 to at least a certain development of the embryo. We may add 

 that in such cases though the muscular walls of the uterus 

 hypertrophy and some changes take place in the uterine mucous 

 membrane, there is no expansion of the cavity, and no true 

 decidua is formed ; the actual presence of the ovum in the cavity 

 is necessary for the full sequence of events, a fact which is inter- 

 esting in reference to the causation of the changes ( 950). 



