Development of the Mammal 295 



this stage has much the same appearance as 

 the blastoderm of the chick at about the six- 

 teenth hour of incubation. So far as is known, 

 it is of about the same size in all mammals. 



At the time of the occurrence of the primi- 

 tive streak, the mesoderm makes its appearance ; 



pk 



feW 



FIG. 92. EMBRYONIC AREA OF A DOG, SHOWING THE PRIMITIVE 

 STREAK. MAGNIFIED 120 DIAMETERS. (From Kollmann, after 

 Bonnet.) 



A, head process, pk* primitive knot. j>r, primitive streak. 



