36 Growth in length of the Vertebrate Embryo 



Thus again we find that in the Mammal and Bird, there is a 

 protogenetic and a deuterogenetic region; and that they corre- 

 spond to one another. 



The protogenetic region gives rise to the fore brain, the mouth, 

 the pharynx, the heart and pericardium, and the deuterogenetic 

 region to all the rest of the animal in both cases. 



Fate of the Primitive Streak. 



We have so far considered the question of the fate of the 

 primitive streak generally. We must now take it more in detail. 



A very interesting paper has recently been published post- 

 humously which was written by E. van Beneden in 1889 on 

 the development of the Rabbit. The work was begun in 1881-2, 

 when the figures were drawn, and was published in 1912. 



The view van Beneden takes is that the first formed didermic 

 condition of the blastocyst of the Amniota is not comparable to 

 the didermic condition of lower vertebrates. 



The inner layer which is usually called the endoderm or 

 hypoblast is, according to van Beneden, not homologous to the 

 endoderm of Amphioxus. It is something else and he calls it 

 Leciihophore. 



As I will shew presently, what van Beneden calls hypoblast is 

 what I should call deuterogenetic endoderm. His lecithophore is 

 what I term protogenetic endoderm and this, according to my way 

 of looking at things, is very clearly comparable to the true endo- 

 derm of the lower chordates. 



Further, van Beneden takes a very different view of the 

 development of the Rabbit from that outlined above. He takes 

 no heed of internal hydrodynamic pressure, or tensions, or any- 

 thing of that kind. To him the cells at the lower pole of an 

 expanding blastodermic vesicle of the Rabbit, are squamous not 

 because of the pressure from within to which they are subjected 

 but he thinks on the contrary that the blastodermic vesicle 

 expands because the cells stretch themselves out. 



He does not regard the cavity of the vesicle as archenteron, 

 nor the inner layer of cells as endoderm and says that "Gastru- 

 lation" does not occur until after the primitive streak has 



