Nucleus as centre of attractive Force 75 



Given these conditions, then the mechanics of the rubber ball 

 model may be reproduced by the living blastula and gastrula. 



The next question to be considered is, can we locate the 

 attraction centre? 



There can be little doubt that if it exists, it is coincident with 

 either the nucleus (Hertwig's law "The nucleus tends to take up 

 a position in the centre of its sphere of influence, i.e., of the 

 protoplasmic mass in which it lies," Przibram), or more probably 

 still, the centrosome of the resting cell. The centrosome is, how- 

 ever, not always easy to see and at any rate it is not often drawn 

 in figures of sections of developing embryos, but the nucleus is 

 usually given, and as the nucleus is never far removed from the 

 centrosome we may for the purpose of the argument take the 

 centre of the nucleus as indicating approximately or perhaps 

 actually the position of the centre of the attractive force. When 

 I first endeavoured to work this out, the only figures available 

 were those of Kowalewski and Hatschek, and those of Hatschek 

 were regarded as authoritative. Now unfortunately for my 

 hypothesis the position shown by Hatschek for the nuclei did 

 not support in any way the contention that the supposed attraction 

 centre of the invaginating cells was more towards the outer surface 

 than the inner surface as was necessary to fulfil the required con- 

 ditions. On the contrary just the opposite was shown, e.g., v. 

 Hatschek, Fig. 21, though not in all cases, e.g., Fig. 20, 2 6. 



But although Hatschek's figures are unsurpassed for clearness 

 and general accuracy, yet as regards the position of nuclei, and of 

 cell walls and in some other respects, they must be regarded as 

 diagrammatic, as recent work on Amphioxus has shown. 



During the last fifteen years sections of the gastrulating Amphi- 

 oxus have been drawn by several authors which are probably more 

 accurate in this respect, because they agree with one another, 

 though disagreeing with Hatschek. They all agree in showing 

 the nuclei of the cells about to invaginate and the invaginating 

 cells as lying close up to the outer surface of the cells, and what 

 is more to the point in most cases distinctly more eccentric than 

 the nuclei of the cells which do not participate in the actual 

 invagination. I allude to the drawings of Sobotta, Samassa, 

 Morgan and Hazen, Wilson, MacBride, Cerfontaine and Legros. 



