SYMBIONTICISM IN KELATION TO HEREDITY 125 



considers the main role to be played by extruded nuclear material 

 in the form of chromidia. 



This process, according to Schaxel, takes place either at an 

 early period of the ovarian egg (fig. 344) or in the blastomeres or 

 their products immediately preceding the conversion of these 

 cells into the differentiated tissue-cells; and the latter process is 

 carefully described by Schaxel in the mesenchyme-cells of an- 

 nelids and other objects. Development thus falls into two well- 

 marked periods, an earlier one in which the general framework 

 of development is established in the egg, and a later one in which 

 more specific differentiations are initiated. Schaxel does not, 

 however, believe that the chromidia are directly transformed into 

 the formed elements of the cytoplasm. They disappear as such, 

 having accomplished the initiation of differentiation and localiza- 

 tion. The nucleus, therefore, initiated differentiation, while the 

 cytoplasmic elements (chondriosomes and others) are the more 

 immediate agents of the process. 



One would like to accept this conception of development and 

 differentiation, which offers so simple a view of the mechanism 

 of the process. Unfortunately evidence concerning the extrusion 

 of chromidia from the nucleus in the manner described by Schaxel 

 is still too conflicting to be accepted without much further in- 

 quiry; and the same must be said concerning the direct origin 

 of formed cytoplasmic elements from extruded fragments of 

 nucleoli. On the other hand, it is certain that a large amount 

 of nuclear material, both liquid and formed, is given off from the 

 germinal vesicle in the prophases of the polar mitoses, and to a 

 less extent in the prophases of other mitoses. Experiment has 

 shown that the material thus set free has a most important 

 physiological effect upon the cytoplasm of the egg; and there is 

 evidence that the escaped material or "residual substance" of the 

 germinal vesicle may contribute directly to the formative material 

 of the egg. In the case of pulmonate gasteropods, for instance, 

 Conklin ('03, '10) has shown that the residual material spreads 

 out over the upper hemisphere of the egg and constitutes in large 

 part the upper stratum from which the ectoblast in these animals 

 takes it origin; and he has found reason for a similar conclusion 

 in the ascidian ('05). F. R. Lillie ('00) has reached a similar result 



