212 RICHARD ASSHETON. 



of the primary centre of growth (the most anterior portion 

 entirely from the primary centre, the most posterior possibly 

 from both). 



The circular area measures transversely 1*2 mm. Therefore 

 we may suppose that at least '1 mm. of the increase in total 

 length of the whole embryonal area may be due to the activity 

 of the primary centre. This leaves '97 mm. to be accounted 

 for. 



If we measure 1*2 mm. (the total length assumed to be due 

 to the primary area of activity) from the anterior end we come 

 to a spot Z. The space between this spot Z. and the anterior 

 end of the primitive streak, A. PS., measures '97 mm. This 

 seems to me to be evidence that this space, Z. to A. PS., is 

 the area which has been added between the stages fig. 6 and 

 fig. 8. The question remains, where have the cells composing 

 this portion of embryonal area originated "^ They must have 

 arisen either (i) from an area of rapid proliferation in front, or 

 (ii) from a general area of proliferation in situ, or (iii) from a 

 general area of proliferation behind. Absolute proof is, as far 

 as I can see, impossible, but I will take each alternative 

 separately, and give my reason for believing that the latter 

 only can be the true solution. 



(i) — (a) From the very first moment of the development 

 the effect of the primary centre of activity is in the ontogeny 

 of the rabbit to produce an embryo of a radially symmetrical 

 figure, whereas the area in question between Z. and A. P. S. 

 shows no sign of a radial symmetry. 



(b) By the time that the area in question is developed, and 

 while it is being developed, there is no sign of one spot from 

 which all pre-existing structures could obtain material. 



For these reasons I think growth does not take place in front. 



(ii) — (a) Growth of the area in question is, comparatively 

 speaking, rapid, but there is no sign of the rapid multiplication 

 of undifferentiated cells, every cell within this area is distinctly 

 either epiblast, hypoblast, or mesoblast. 



[b) This area continues to grow rapidly in length, and the 

 "mesoblastic somites," which are a characteristic feature of 



