i62 CYTOLOGY chap. 



and Fuchs, 1914) are found, like all other hybrids, to exhibit certain 

 characters of one parent and certain of the other. MacBride (1911) 

 examined larvae from the cross Echinocardium 9 x Echinus ^^ at a much 

 later stage of development than has usually been employed by workers 

 on Echinoderm hybrids, and also found them to exhibit characteristics 

 of both parents. 



Summing up, the predominance of maternal characters sometimes 

 found in the young stages of hybrids, mostly between distantly related 

 forms, is reconciled with the equaUty of inheritance from male and 

 female parents which is the general rule in those vastly more numerous 

 hybrids which have been studied in the adult condition, by (i) the 

 observation of elimination of paternal chromatin from the hybrid nuclei ; 



(2) the hypothesis of the impotence of idioplasm in a foreign cytoplasm ; 



(3) (in the case of very early embryos) by the fact that the first 

 stages of development consist in the remodelling of the substance of 

 the Qgg into the embryo, without the formation of any new substance 

 under the influence of the zygote nucleus. ^ 



B. THE PROCESS OF MITOSIS AND ITS IMPLICATIONS. THE 

 FUNCTIONAL DIFFERENTIATION OF THE CHROMOSOMES 



The compUcated processes of mitosis — the rearrangement of the 

 apparently irregularly distributed chromatin of the resting nucleus into 

 linear series forming long chromatin threads, the longitudinal fission of 

 these involving the division of each element of the series, and the 

 separation of the daughter threads in anaphase, one of each pair going 

 to the one daughter cell and the other to the other — are obviously adapted 

 to ensure the accurate division and distribution of a mass consisting of 

 a number of differentiated elements, such as we must suppose the 

 hereditary substance to be. On the other hand, nothing of the kind is 

 recognizable in the division of the cytoplasm of a mother cell into two 

 daughter cells. 



This lack of a distributing mechanism for the cytoplasm at cell 

 division argues" an essential homogeneity of the cytoplasm. This does 

 not imply that it may not contain a mixture of different substances 

 nor possess a definite structure, but that it is not composed of locahzed 

 elements of differentiated function essential for the Hfe of the organism, 

 and incapable of being regenerated if lost. We must quahfy this thesis 

 by an apparent exception, of a parallel nature to that already made 

 to the thesis of equality of inheritance from the two parents, which we 



1 See also the discussion oa " organ-forming substances," p. 188, 



