Ill DIFFERENTIATION 105 



nucleus there it becomes irregularly divided in the next or 

 some following cleavage. Partial thelykaryosis therefore 

 never occurs. Hence also ultimately no cell in the body can 

 have the full set of paternal chromosomes. Occasionally com- 

 posite spindles are found in these eggs that is, between the 

 female monaster and an undivided male centre. 



There seems little room for doubt, when all these results are 

 considered, that the characters of the skeleton and body -shape 

 at least, by which one genus of pluteus is distinguishable from 

 another, are transmissible from the male as well as from the 

 female parent, and therefore by the nucleus. 



The experiments of others, while they lead to the like 

 conclusion, illustrate also the difficulty of discovering the 

 causes which determine whether the characters transmitted 

 by the nuclei shall or shall not become manifest in the 

 hybrid. 



Thus MacBride, crossing Echinocardium cordatum ? with 

 Echinus esculentus o*, finds that the hybrid larva is smaller 

 than that of either pure type, that the aboral spike charac- 

 teristic of the skeleton of the pluteus of the female parent is 

 absent, but that a male parental character, the bending in 

 of the ends of the apical rods, is present, while the skeleton of 

 the post-oral (anal) arms may be of the maternal, of the 

 paternal or of an intermediate type. 



Shearer, De Morgan, and Fuchs employ not the highly 

 variable skeletal characters of the early plutei, but the stable 

 features of the later larvae, such as the posterior pedicellaria 

 found in Echinus esculentus and acutus but not in Echinus 

 miliaris, the posterior epaulettes found in the two former but 

 not in the latter, and the green pigment found in miliaris but 

 not in the other two. When esculentus or acutus is crossed 

 with miliaris the posterior epaulette appears, whether the 

 parental form which possesses it is used as a male or female, 

 while the green pigment of miliaris is not developed by the 

 hybrid, whether this form has been used as male or female. 

 This result, confirmed by Debaisieux, is not however invari- 

 able, for on previous occasions the authors referred to had 

 found that the characters in question were always transmitted 



