LIMITS OF I ) i \ isi i; 1 1, IT v OF LIVING MATTER 337 



one-eighth <>f the mass of the entire egg (nucleus ]>li<x pro- 

 toplasm (. 



c) The amount of substance necessary for the formation 

 of a blastnla is much smaller than that necessary for the 

 formation of a pluteus; for the formation of a gastrula more 

 substance is probably required than for the formation of a 

 blastula. 



<l) It does not matter which position the fragments of an 

 (M^r of Arbacia occupied in the intact egg; so far as divisi- 

 bility is concerned, the protoplasm of the Arbacia egg can 

 certainly be considered as isotropic. 



<") Since the limits of divisibility are almost the same in 

 the unsegmented egg as in the first stages of segmentation 

 (the thirty-two-cell stage included), it follows that (a) no 

 qualitative changes occur in the egg during the early stages 

 of segmentation which restrict the development of fragments 

 of the egg, and that (ft) the individual cleavage cells, so far 

 as the limits of divisibility of the substance of the egg are 

 concerned, may be considered as equal. (In other respects, 

 however, differences may exist between the individual cleav- 

 age cells.) 



