MAMMALIA. 4^5 



J shells and yolk ; and the structure of the fcetal membranes, 

 so far as is known, does not essentially differ from that in 

 Sauropsida, This fact and others appear to justify zoologists 

 in assuming that the present-day mammals are descended 

 from ancestors which in these respects resemble the mono- 

 tremes. In other words, the change from an oviparous to 

 a viviparous habit is supposed to account for the differences 

 in structure and function. We must assume that gradually 

 the egg was retained for a longer period before being laid. 

 The serosa then became an organ of attachment to retain 

 the egg, the shell having become superfluous. The embryo 

 was thus nourished by albumen from the uterine glands. 



Thus was instituted a habit of ovoviviparity in which the 

 young, was hatched inside the mother. The interchange of 

 blood-elements between the blood-vessels of the widely dis- 

 tended yolk-sac and the enveloping maternal tissue was 

 inevitable, and the yolk being no longer required it com- 

 menced to atrophy. Thus the metatherian condition is 

 reached in which the yolk-sac placenta is functional and the 

 allantois becomes vestigial. 



If, however, the allantoic arteries and veins, as well as 

 the vitelline, become connected with the uterus, the same 

 atrophy of yolk results, and the allantois eventually replaces 

 the yolk sac as a placental organ. To its former function of 

 respiration is therefore added that of nutrition. 



The removal of the yolk explains the reversion to a total 

 equal segmentation and the formation of a " metagastrula," 

 whereas the enormous increase in size of the egg on entry 

 into the uterus may be explained as being due to the 

 necessity for the ^gg being of the same large size as it 

 originally was when there was much yolk, the large surface 

 being required both for absorption and mechanical attach- 

 ment. 



We may briefly summarise the development of a mammal 

 as follows : — 



T. Discharge of ovum from Graafian follicle of ovary and 

 passage into Fallopian tube. 



2. Maturation and fertilisation in Fallopian tube, followed 

 by total equal segmentation and invagination of hypoblast to 

 form metao^astrula. 



