78 Geometrical relation of Nuclei 



This is very prettily shown by Wilson's experiments on the 

 blastomeres of Amphioxus. Wilson found that by shaking apart 

 the blastomeres formed by the first cleavage furrow, each separated 

 blastomere developed normally and became a perfect blastula, 

 gastrula, and metameric embryo, but half the size of a normal one. 

 So also each of the first four segments became perfect gastrulae 

 and free swimming embryos. Now in both these cases the plane 

 of division is vertical and median and divides the egg in such a 

 way that the relative distribution of the protoplasm and yolk 

 between upper and lower poles is undisturbed. 



But a very different result was obtained by the segments of the 

 eight cell stage. In this case the eight cells are derived from the 

 four cells by an equatorial or horizontal division, which entirely 

 upsets the proportion of yolk to protoplasm. Each of these eight 

 segments isolated by Wilson continued to divide, but since their 

 composition was utterly unlike the composition of the whole ovum 

 of Amphioxus, each became an embryo unlike an Amphioxus 

 embryo, and although a few became blastulae not one ever became 

 a gastrula. Wilson says: "None of the ^ embryos, as I believe, 

 are capable of full development. I have isolated a considerable 

 number of the ^ blastomeres, and of the later embryos of the 

 various types (i.e., '(a) perfectly flat plates of cells, (6) more or 

 less curved plates and (c) blastulas one-eighth the normal size, 

 either closed or with a pore at one side'), and have observed 

 hundreds of all stages without once obtaining a gastrula." 



Morgan a few years later repeated these experiments and he 

 gives on the whole a similar, but not so dogmatic an account as 

 Wilson's. Thus he says some of the blastomeres fail to gastrulate 

 while a few of the blastomeres form blastulae which "partially 

 gastrulate." "Many of the ^ blastomeres develop into hollow, 

 swimming blastulae, which swim around for several hours after 

 the normal blastula has gastrulated. Some of these blastulae 

 flatten at one pole as though about to gastrulate, but do not seem 

 to be able to develop further." 



The shaking of the eggs however seems to cause abnormalities 

 even in the whole ova. 



I think we may safely conclude that whereas the rule is that 

 and J blastomeres develop as the whole ovum normally does, 



