32 Growth in length of the Vertebrate Embryo 



proliferation of cells of the outer layer of the hind part, probably 

 of the actual edge of the embryonal area. 



The proliferation is from the under surface. But we must 

 presume also an addition of cellular units to all tissues in contact 

 with the proliferating area. The cells which are proliferated 

 within give rise chiefly to mesoderm and anteriorly also to noto- 

 chord, that is to say deuterogenetic notochord. We will return to 

 this point later. 



There can be no doubt that here is an area of intense cell 

 divisional activity. 



This centre of activity has apparently arisen suddenly. It is 

 close to but not coincident with the centre of the primary centre, 

 which we may take as being practically in the centre of the circular 

 embryonal area. 



Now what must be the effect of the counter action of these 

 two centres? We must consider them for the moment apart 

 altogether from any phylogenetic point of view. Here we have 

 two actively productive centres of tissue, one of which is especially 

 active. They lie on the same plane. They are subjected from 

 within to an ever increasing hydrostatical and therefore a hydro- 

 dynamical pressure. Surely the result must be that they tend to 

 elongate along the lines joining their respective centres. 



If one area is more yielding than the other then that area will 

 be elongated the most. 



What are the conditions here? Inside there is an increasing 

 hydrodynamic agent. This is shown by the way in which the 

 albumen layer is kept taut and becomes thinner, and later it is 

 seen by the expansion of the uterine wall. Therefore the cells 

 are stretched also. But where there is also a tendency to expansion 

 due to cell multiplication there will be an additional tendency 

 for the cells to stretch. 



Again if the cell multiplication is greater in one area than in 

 the other, the area in which the cell multiplication is greatest will 

 give way more easily than the other. 



The deuterogenetic area seems to be an area in which cell 

 multiplication is very intense, and therefore we find that this part 

 expands more markedly than the protogenetic area. 



The result is that the posterior half of the embryonal area is 



