124 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



tivities of invisible, organized, "pangens" (analogous in some 

 respects to Darwin's "gemmules") which migrate from the 

 nucleus into the cytosome and in large measure build up the 

 active cytoplasmic substance. A view similar in all its essen- 

 tials was subsequently adopted by Weismann, 0, Hertwig and 

 other writers. More accessible to investigation are hypotheses 

 which assume differentiation to be effected by the transforma- 

 tion of visible cytoplasmic granules or other definite bodies, 

 arising either by migration from the nucleus or independently in 

 the cytosome; and these views are based to some extent on 

 direct cytological observations. The questions that here arise 

 (p. 720) evidently apply alike to the ovum before its cleavage 

 begins, to the early blastomeres and the embryonic cells derived 

 from them, and to the tissue-cells in so far as they may be capable 

 of further differentiation (by differentiation, redifferentiation and 

 the like). 



Existing knowledge of this subject is still too fragmentary and 

 discordant to offer a sufficient basis for adequate discussion. At 

 one extreme are authors who have ascribed cytoplasmic differen- 

 tiations to the activities of chromidia, extruded from the nucleus 

 of the egg. This view remains to say the least, very doubtful, 

 though there is a certain amount of evidence that certain of the 

 formed cell-components may arise from extruded nucleolar frag- 

 ments. At the opposite extreme is the view of Altmann, Benda 

 and Moves that the leading role in differentiation is played by 

 granules (in particular mitochondria and other forms of chon- 

 driosomes) that are of purely cytoplasmic origin, and which 

 possibly may arise by the growth or division of preexisting bodies 

 of the same kind. 



An intermediate position between these opposite extremes is 

 taken by Schaxel in an interesting series of studies on the de- 

 velopment of coelenterates, echinoderms and annelids. This 

 observer accepts up to a certain point the conclusions of both 

 sides, recognizing the existence of both chromidia and chondrio- 

 somes in the egg and the tissue-cells, and ascribing to both an 

 important role in differentiation. Schaxel admits that the 

 chondriosomes or "plastosomes" may become directly trans- 

 formed into differentiated components of the tissue-cells but 



