156 



Nachdruck verboten. 



Oll Grrowth Centres in Vertebrate Embryos. 



B}' Richard Asshbton. 



With 9 Figures. 



(Schluß.) 



If the egg of Rana temporaria is taken and at the moment of the 

 first appearance of the dorsal lip of the blastopore a sable hair is in- 

 serted so as to penetrate the mid dorsal line 0,5 mm in front of the 

 dorsal lip, then, when the egg hatches, a fairly accurate mark between 

 the primary area of cell production, and the secondary area of cell pro- 

 duction will be obtained. If, as the blastopore forms its lateral 

 and ventral lips, the experiment is performed in a similar way on the 

 sides and ventral surface, a fairly accurate determination of the two 

 areas, primary and secondary, will result, A consideration of the ex- 

 perimental and anatomical evidence will shew that the term noto- 

 genesis represents only half the truth, for this secondary centre of cell 

 proliferation adds new material not only to the back, but to the sides 

 and to the ventral regions of the pre-existing organism, which was the 

 result of the activity of the primary centre. 



It is true that apparently more material is added to the dorsal 

 surface than to the ventral. But is it not more apparent than real? 

 For, 1) the ventral surface is swollen out by the yolk accumulation 

 and so loses its correct proportion with reference to the dorsal regions ; 

 2) the dorsal part of the proliferating area comes into action long be- 

 fore the lateral and ventral parts. It is partly real no doubt, as the 

 dorsal part ultimately grows out vigorously to form the tail, while the 

 ventral part soon grows less vigorous and dies out. 



The Figures 1 — 6 represent actual experiments of this kind upon 

 the eggs of Rana temporaria. 



In Fig. 1 A the very earliest sign of the blastopore may be seen 

 and at a distance of 0,5 mm in front of this pit the position in which 

 the sable hair was inserted is indicated also. 



In Fig. 1 B the neural folds are formed and the sable hair oc- 

 cupied in the experiment the position marked on the diagram. Fig. 1 C 

 is a horizontal section of the specimen at a later stage when the em- 

 bryo measured about SVg or 4 mm and in this the position of the sable 



