EARLY MAMMALIAN DEVELOPMENT. 



93 



bedded in the uterine mucosa. As soon as the blood is present the entodermal 

 cells, and also the cells of the ectoplacental cone, take on the vacuolated 

 appearance and haemoglobin can be demonstrated in the cytoplasm. On 

 the whole, however, the mechanism for nourishing the developing ovum 

 appears to lack the efficiency of that in other mammals. On this account 

 Sobotta has presented the conclusion that the inversion of the germ layers 

 is an attempt on the part of the developing organism to increase the ento- 

 dermal surface and to put the layer of entoderm in closer contact with the 

 source of nutriment, that is, the maternal blood. 



Mesoderm. Returning now to the bat, it has already been noted (page 

 89) that the embryonic disk is nearly flat and composed of ectoderm and 



Embryonic disk 

 Hensen's node 



Peristomal 

 mescderm 



FIG. 64. Embryonic disk of dog. Bonnet. The letters and figures on the right (Si-S 4 ) indicate 



planes of sections shown in Fig. ; 5 



entoderm (Fig. 59, c). In the dog the disk is of similar form and construc- 

 tion, but has no amniotic cavity over it. In the dog's disk then a linear 

 opacity appears which extends about two-thirds of the way across the disk 

 (Fig. 64). Obviously this band represents the primitive streak. In cross 

 section the primitive streak is seen to be composed of ectoderm and ento- 

 derm fused with an intermediate layer of mesoderm (Fig. 65, S$ and S^). 

 The arrangement of the layers here corresponds exactly with that in the bird. 

 (Compare Fig. 47.) The mesoderm extends laterally between ectoderm 

 and entoderm as a number of more or less scattered cells. Sections taken 

 in front of the primitive streak show the entoderm and mesoderm fused to- 



