46 Growth in length of the Vertebrate Embryo 



The most important feature is perhaps one which it is difficult 

 to see in the chick in surface view. 



This is the fact that the primitive streak is clearly an area of 

 intense cell proliferation and that cells are being continually 

 budded off from its whole length into the space between the 

 ectoderm and endoderm ; and that these are spreading out to the 

 sides and behind and in front as well. This is also mesoderm 

 but mesoderm having a different origin to that which was 

 mentioned before, as occurring in the anterior region of the 

 blastoderm, cf. Fig. 25^4. 



Experimental proof. 



It is possible to apply just the same tests experimentally to 

 the eggs of Birds as have been already described for the eggs 

 of Frogs. 



The Bird's egg may be opened by removal of a small piece 

 of the shell, a mark made on the blastoderm, and the shell 

 replaced. 



Or the whole egg, yolk and albumen, may be turned out into 

 a suitably sized vessel, and the yolk kept down below the surface 

 of the albumen by means of a glass ring ; the bristle or hair can 

 then be inserted and the vessel, after being covered with parch- 

 ment or even with a glass lid, can be placed in an incubator, 

 where development will proceed for two or three days almost 

 normally. 



By this means it will be found that part of the animal is 

 formed in front of the primitive streak, that is to say by proto- 

 genesis, and part by the activity of the primitive streak, that is 

 to say by deuterogenesis, and that the two areas correspond to 

 the two parts derived from those centres in the Amphibian. 



