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 bases are con- 
trolled by the cytoplasm. However, zz the escape of large quantities of nuclear 
material anto the cell body and the formation there of specific protoplasmic sub- 
stances we have a possible mechanism for the nuclear control of the cytoplasm, and 
when, as tn the case of the ascidians and fresh water gasteropods, these substances 
are definitely localized tn the egg, and can be traced throughout the development 
until they enter into the formation of particular portions of the embryo, a specific 
mechanism for the nuclear control of development ts at hand, and the manner of 
harmonizing the facts of cytoplasmic organization with the nuclear inherttance 
theory 15 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 back to the cytoplasm as are taken up from it; on the contrary 
we know that some of the substances which escape from the nucleus (е. 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 ерігепейсайу. 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 oógenesis, 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 їп 
the ovary may, under the conditions of a free cell, begin another series of differen- 
tiations which lead to the production of an organism. 
