3/2 CLASSIFICATION OF VERTEBRATES. 



The development of the mammals pursues two distinct types. 

 In the monotremes there are eggs which are laid, and which un- 

 dergo their development outside the body of the mother, as do 

 those of birds and most reptiles. These eggs are large (about 

 two centimetres in diameter). They have a large yolk, and the 

 segmentation is restricted to a small portion of it, just as is the 

 case in the sauropsida ; i.e., they are meroblastic. 



In all other mammals the egg is much smaller, even micro- 

 scopic in size, and the early stages of development are passed 

 inside the mother, the young being born alive. These smaller 

 eggs undergo a total segmentation, all parts dividing ; i.e., they 

 are holoblastic. During this process the egg increases greatly 

 in size by the absorption of fluid which fills the central cavity. 

 As a result the egg is converted into a large sphere (blastula) 

 covered by a single layer of cells except at one pole, where 

 there are a number of ' inner cell-mass cells ' beneath the others. 

 Concerning these layers there is much difference of opinion, 

 due to the great difficulty surrounding the subject. According 

 to one view the outer cells are ectoderm, the inner entoderm ; 

 according to another the inner cell-mass is ectoderm, the outer 

 entoderm, while a third view sees both ectoderm and entoderm 

 in the inner cell-mass. Certain it is that the region of the inner 

 cell-mass eventually becomes two-layered, and later the embryo 

 is outlined here, only a portion of the blastula being utilized in 

 its formation, the rest forming a yolk sac, in the walls of which 

 omphalomeseraic vessels are developed later, although no yolk 

 occurs. 



This development of a complete yolk apparatus, as well as 

 several other peculiarities, is to be explained upon the hypothesis 

 that the mammals have descended from forms like reptiles or 

 amphibia which were oviparous, and the embryo had to de- 

 pend upon the food stored up in the yolk. Subsequently, as a 

 result of an internal development and a supply of nourishment 

 from the mother, the yolk was lost ; but heredity has caused 

 certain features not incompatible with uterine development to 

 be retained. The mechanism by which this nourishment from 

 maternal sources is transferred to the embryo has now to be 

 outlined. 



