157 



is seen to be on the boundary line between the fore and hind brain, 

 and just in front of the anterior end of the notochord. 



Clearly from this one must conclude that the whole of the fore 

 brain belongs to the primary growth centre. Since, however, the sable 



olf C 



Sable 



3rcl Som. 



Fig 1. Rana temporaria. A Dorsal 

 Sable surface of the egg. Bl earliest stage in 

 formation of the blastopoi'e, Sable the spot 

 at which the bristle was inserted. B Dorsal 

 surface of the same egg after the appearance 

 of the neural folds. C Horizontal section 

 taken some days later, mi auditory vesicle, 

 fb fore brain, olf olfactory epithelium, ph 

 pharynx, Srd Som. third mesoblastic somite. 



was inserted 0,5 mm in front of the dorsal lip of the blastopore, prob- 

 ably we must include also some portion posterior to the fore brain 

 as belonging to the primary growth centre. 



It is not possible to obtain satisfactory results if the sable is 

 placed at a point nearer the blastopore lip than 5 mm: it causes too 

 much disturbance of the normal processes. It is unlikely that there 

 should be an absolutely hard and fast line between the influences of 

 the two growth centres. But this much we can say — which is borne 

 out by experiments on other vertebrate embryos — that as regards the 

 more dorsal surface, everything in front of the mid brain is certainly 

 due to the primary growth centre. Everything posterior to and in- 

 cluding the first pair of mesoblastic somites is certainly due to the 

 secondary growth centre. The division between the two influences lies 

 between these points — more probably the influences overlap. 



In Fig. 2 a similar experiment is recorded. Here the position 

 of the sable as seen in side view is shewn, in C as it was inserted 

 immediately on the appearance of the blastopore as a little elongated 

 groove; in B and A in section at a later stage of development. 



