MAMMALIA, 485 
shells and yolk ; and the structure of the foetal 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 egg 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 :— 
1. 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 metagastrula. 
