xv] TRANSMISSION BY CYTOPLASM 245 



is a purely maternal character, and apparently dependent 

 on the egg-cytoplasm, but BOVERI maintains that on this 

 feature the spermatozoon has at least some effect, the 

 number being greater in a hybrid in which the paternal 

 species has more than the maternal. 



Another kind of cytoplasmic influence may arise from 

 the direct transference to the offspring of structures or sub- 

 tances present in the egg-cytoplasm. The eggs of the Sea- 

 urchin Arbacia contain much red pigment, and in conse- 

 quence all hybrid larvae produced from Arbacia eggs are 

 pigmented, while those produced from the converse cross 

 may be colourless. And in plants the chloroplasts are trans- 

 mitted only in this way; if the cytoplasm of the egg-cell 

 contains chloroplasts these are present and multiply in the 

 cells of the embryo, but an egg-cell lacking chloroplasts 

 gives rise to offspring that are totally without chlorophyll. 



Apart from these rather special instances of characters 

 dependent exclusively on the cytoplasm or its inclusions, 

 there is evidence of a more general character that the cyto- 

 plasm affects development independently of the chromo- 

 somes, although it may be that in consequence of the inter- 

 action between chromosomes and cytoplasm, some of these 

 effects may have their origin in the chromatin-cytoplasm re- 

 lation rather than in the cytoplasm itself. In WEISMANN'S 

 hypothesis of the germ-plasm it was supposed that all differ- 

 entiation of cells or tissues arose from differential cell- 

 divisions in which the chromosomes of the two daughter 

 nuclei were in some way unlike. But except in the few cases 

 of germ-cell differentiation referred to in Chapter XII, there 

 is little or no evidence for the existence of such differential 

 nuclear divisions. One of the chief characteristics of the 

 chromosomes is that they appear to divide in mitosis into 

 longitudinal halves which are precisely similar to each other, 



