VI 



ECHINODERM HYBRIDS 



161 



circumstances it is not surprising that the embryos were of the purely 

 maternal type. 



(b) We have already just emphasized that the nucleus can only 

 exert its morphogenetic function through the cytoplasm. Now the 

 earlier stages of development proceed with little or no increase of cyto- 

 plasm. They consist indeed largely in the remodelling of the substance 

 of the egg into that of the embryo. Hence in these very early stages 

 the nucleus of the zygote has little opportunity to exert its morphogenetic 

 function. This can only get full scope after the embryo has begun to 

 form new cytoplasm by assimilation either of food substances supplied 

 from without, or of reserve food material stored within it. Thus the 



FIG. 73. 



Diagram of the fertilization of the egg of Echinus microtuberculatus by the sperm of Mytilus galloprovin- 

 cialis. (Kupelweiser, A. E-M., 1909.) A, B, nucleus, preceded by aster, approaching $; C, D, first cleavage 

 nucleus formed entirely from the 9 nucleus, <$ nucleus unchanged ; E, 2-cell stage. The 6" nucleus, still 

 unchanged, lies inert in that one of the blastomeres to which cell division has chanced to relegate it. 



very early form of the embryo must be determined to a large extent 

 by the physical constitution of the egg cytoplasm. 



The resemblance to the maternal species during cleavage, gastrulation, 

 etc., which is brought about by the purely mechanical factors of size 

 and number of yolk granules, viscosity of cytoplasm, etc., could only 

 by a confusion of ideas be brought into the same category as the 

 resemblance to parents due to the presence of the living, self-repro- 

 ducing idioplasm. To apply the word " Heredity " to both these cases 

 would be to confuse the meanings of the term as used biologically and 

 socially, as is done when a child who has been infected in utero with 

 Spirochaeta pallida is said to have inherited the disease of syphilis. 



Finally, it must be mentioned that the only Echinoderm hybrids 

 which have been examined in the adult condition, namely, the offspring 

 of the crosses Echinus miliaris <j> x Echinus acutus $ (Shearer, de Morgan 



