THE PHYSICAL BASIS OF HEREDITY, 



175 



generates. A few moments after the sperm has entered 

 a system of radiations appears around the middle piece, 

 which develops into an aster surrounding the centrosome 

 of the sperm (Fig. 10, B). The sperm nucleus swells 

 up and rapidly increases in size, its chromatin changing 

 from the compact condition in which it is arranged 

 in the sperm head to a reticulate condition (Fig. 10, C). 

 The chromatin reticulum of the ^%g nucleus becomes 

 also more clearly visible. Sperm aster and sperm nu- 

 cleus now move in toward the egg nucleus, the aster 

 usually preceding. As the nuclei approach the sperm 

 nucleus increases still more in size until it becomes in- 

 distinguishable from the egg nucleus (Fig. 10, C). The 

 chromatin network of each now breaks up into a number 

 of chromosomes, one half of the number found in the 

 somatic cells, and the nuclei come into contact, fusing 

 together in some cases. In the sea urchin. Echinus, the 

 number of chromosomes is eighteen, nine would there- 

 fore be found in the germ nuclei ; for the sake of clear- 

 ness and simplicity but two are represented in the dia- 

 gram, those of the sperm nucleus being slightly shaded 

 while those of the egg nucleus are black. The centro- 



_, some divides together with its aster 



Cleavage. 



{r\g. 10, JJ), the two daughter centro- 



somes move apart to opposite poles of the tgg, and the 

 typical amphaster of cell division is formed (Fig. 10,^), 

 the nuclear membranes disappearing and the chromo- 

 somes being drawn together into the equatorial plate 

 where each splits longitudinally. The halves are drawn 

 by the mantle fibrils toward the opposite poles and the 

 Qgg divides transversely into two cells (Fig. 10, F). This 

 process of division is repeated continuously in each of 

 the resulting generations of cells, and from the mass 

 of cells thus formed develops the new organism. Each 



cell in the two-celled stage has received half of its 

 13 



