54 HOW ANIMALS DEVELOP 



in the front end of the primitive streak. The difference 

 of opinion shows how doubtful the question is : in 

 some ways one comparison is more informative, in 

 some ways the other. In both cases in front of the 



Fig. 14. — Map of the presumptive areas in the chick, 

 in the late primitive-streak stage, aged 18 hours. Only 

 the transparent area is shown. The arrows show the 

 directions of movement and the primitive streak is 

 drawn black. Widely spaced vertical lines, skin; close 

 horizontal lines, neural plate; dotted, mesoderm. The 

 endoderm lies underneath the whole surface 



blastopore there is a zone of presumptive mesoderm 

 and outside that a zone of presumptive neural plate, 

 just as there is in the newt or lamprey. In the map of 

 the later stage (Fig. 14) part of the mesoderm has 

 already disappeared inside. 



Mammals 



We must now go on to consider the development of 

 mammals, but it will only be possible to give a very 



