ORGANIZATION AND CELL-LINEAGE OF ASCIADIN EGG. 101 



This truly remarkable condition in which considerable portions of the cyto- 

 plasm are traceable to the nucleus is of the utmost theoretical importance. From 

 all sides the evidence has been accumulating that the chromosomes are the seat 

 of the inheritance material, until now this theory practically amounts to a demon- 

 stration. On the other hand, all students of the early history of the egg have 

 observed that the earliest visible differentiations occur in the cytoplasm, and that the 

 position, size and quality of the cleavage cells and of various organ liases are con- 

 trolled by the cytoplasm. However, in the escape of large quantities of nuclear 

 material into the cell body and the /or/nation there of specific protoplasmic sub- 

 stances we have a possible mechanism for the nuclear control of the cytoplasm, and 

 when, as in the case of the ascidians and fresh water gastcropods, these substances 

 are definitely localized in the egg, and ca?i be traced throughout the development 

 until they cuter into the formation of particular portions of the embryo, a specific 

 mechanism for the nuclear control of development is at hand, and the manner of 

 harmonizing the pacts of cytoplasmic organization with the nuclear inheritance 

 theory is clearly indicated. 



Of course substances which enter the nucleus and contribute to its growth 

 must reach it through the cytoplasm, but this does not signify that the same sub- 

 stances are given hack to the cytoplasm as are taken up from it ; on the contrary 

 we know that some of the substances which escape from the nucleus (e. g., oxy- 

 chromatin) are not identical with those which enter it. Considering the necessity 

 of the nucleus in assimilation and regeneration, it seems most likely that differentia- 

 tions of the cytoplasm proceed in the first instance from the nucleus; and, indeed, 

 in the case of the egg cell, some of the important cytoplasmic substances can 

 be actually seen to come from the nucleus. This does not indicate that these sub- 

 stances exist from the beginning in the nucleus; on the contrary there is direct and 

 visible evidence that they arise epigenetically. Such epigenesis, however, does not 

 signify lack of primary organization; on the other hand all the evidence favors the 

 view that back of the organization of the cytoplasm is the organization of the 

 chromosomes, which is definite, determinate and primary. 



What has been said with regard to the genesis of the different substances of 

 the cytoplasm applies in the main to their localization. It is evident that this 

 localization is progressive, and that it arises epigenetically. But though we may 

 push back this localization to earlier and earlier stages and to simpler and simpler 

 forms we cannot entirely do away with it, even though it may be traced to polarity 

 and chemotropism. Some basis of localization must be present in the earliest 

 stages of the oogenesis, but this may possibly be little more than is found in the 

 body cells in general. It does not seem improbable that the differentiations and 

 localizations of the ovocyte and of the tissue cells are comparable in their manner 

 of origin. The most remarkable difference between the two is that the tissue 

 cells having reached the limit of their differentiation are incapable of further 

 development whereas the egg cell having reached the limit of its differentiation in 

 the ovary may. under the conditions of a free cell, begin another scries of differen- 

 tiations which lead to the production of an organism. 



