The Germinal Spot in Echinodcrm Eggs. 7 



probably similar number for Asterias vulgaris. Since the number is known 

 to be less than 36 in some forms it is reasonable to expect that it may be more 

 in other forms. Again, it is possible that the quadripartite masses may 

 represent only parts of final chromosomes. But whatever the value of these 

 bodies in terms of a univalent chromosome it is clear that they originated 

 as the result of the fragmentation of the nucleolus. Nor can the chromo- 

 somes originate from elsewhere, for there appear no other chromatic 

 structures anywhere in the history of the growth-period, except the occa- 

 sional beaded threads which are not invariably present and have the same 

 elongate thread-like appearance in the latest as in the earliest stages of de- 

 velopment. These threads are very similar to those occasionally seen in the 

 growing egg of Asterias forbesii where they are known not to be chromo- 

 somes, but where they probably represent streams of chromatin material in 

 transit to or from the nucleolus. 



FIG. 5. a, ovum of Echinaster at culmination of growth-period. Nucleus shrunken away 

 from wall, leaving a lacuna partly filled by a homogeneous coagulum. Nucleoplasm 

 very finely granular and pale-staining. Throughout it are scattered many chromatic 

 bodies composed of one, two, three, or four globes. Many assume a typical tetrad- 

 shape, b shows such a one magnified. X 1500. The cytoplasm is light-staining and 

 typically alveolar. < 160. 



Again, it might be objected that these bodies are products of nucleolar 

 degeneration. But in reply it can be said that all the eggs are of similar 

 constitution and that no other portion of the individual eggs gives any evi- 

 dence of degeneration. Furthermore, it is possible to find hundreds of 

 transition stages between those illustrated in figures I and 5, and the change 

 is a progressive one from the very first intimation of fragmentation to the 



