222 EMBRYOLOGY. 



For the explanation of these conditions we must have recourse 

 to an hypothesis which can be formulated about as follows : 



The Mammalia must have descended from animals which possessed 

 large eggs with abundant yolk, which were oviparous, and in ivhich 

 consequently the embryonic membranes were developed in the same way 

 as in Reptiles and Birds. The loss of the yolk-contents from the eggs 

 of these animals must have been a supplementary event, which began 

 at the time when the eggs were no longer deposited outside, but were 



Tig 129. Diagram of the foetal membranes of a Mammal, after TURNER. 



pc, Zona pellucida with villi (prochorion) ; sz, serous membrane ; E, outer germ-layer of the 

 embryo ; am, amnion ; AC, anmiotic cavity ; M, middle germ-layer of the embryo ; H, inner 

 germ-layer of the same; UV, yolk-sac (vesicaumbilica'is); ALC, allantoic cavity ; al, allantois. 



developed in the uterus. For by this change there was found a new 

 and more productive, because unlimited, source of nourishment for the 

 developing germ in substances which were secreted by the walls of the 

 uterus from the maternal blood. There was therefore no more need of 

 a dower of yolk. But the enveloping structures, which were originally 

 called into existence by the presence of yolk-contents in the eggs, 

 were retained, because they were still of use in many other relations, 

 and because, through a change of function, they became subservient 

 to uterine nourishment and correspondingly underwent changes. 



