ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 93 
C. CYTOPLASMIC LOCALIZATION, 
l. Localization in Cleavage Stages. 
That there is a specification and localization of those portions of the proto- 
plasm of the egg which are destined in development to give rise to definite organs 
has been repeatedly affirmed and denied since His first propounded the doctrine of 
* organ forming germ regions" in 1874. At first this doctrine took the form of a 
mental projection of the early embryonic organs back upon the unsegmented egg. 
Later the study of cell-lineage showed that definite organs of the larva or adult 
arose from definite blastomeres, which in turn came from definite portions of the 
unsegmented egg. But although it was thus possible to map out the cleavage cells 
and the unsegmented egg into regions corresponding to certain organs of the embryo, 
it was not usually possible to show that these regions were visibly different from one 
another. Nevertheless the fact that certain blastomeres constantly gave rise to 
certain parts, and that other blastomeres developed very differently and gave rise 
to other parts, led students of cell-lineage generally to the view that there must be 
some protoplasmie difference between such blastomeres, though it might not be 
directly visible. 
On the other hand were those who maintained that the protoplasm of the early 
cleavage stages was undifferentiated and that specifications which determined the 
fate of these cells arose only at a later period and under the influence of environ- 
mental or extrinsic conditions, such as mutual interaction between the cells, position 
in the.developing embryo, etc. Such views were maintained оп the ground of 
experimental work, especially that of Driesch, Hertwig, Morgan, Wilson and others, 
but it should not be forgotten that the experimental work of Roux furnished 
important evidence in favor of the independent differentiation, “ Se/bs/differenzr- 
rung, of different blastomeres, 
Thus while the study of cell-lineage showed conclusively that certain cells 
were destined in the course of normal development to give rise to certain organs 
and that the individual blastomeres were more or less differentiated from one another, 
the results of experimental work showed that in many animals individual cleavage 
cells were capable of giving rise to an entire embryo, and it was, therefore, affirmed 
by some investigators that these cells could not be differentiated for any particular 
end. Inasmuch as these facts of cell-lineage and of experimental embryology were 
well established, it was only possible to harmonize these discordant results by some 
form of interpretation. This was undertaken from two different standpoints: (1) 
It was affirmed that the early cleavage cells were not really differentiated for any 
‘specific end and that each might develop into any part of the embryo; if in any 
case certain parts or organs came from certain blastomeres it was due merely to the 
“continuity of development" (Hertwig, O., 1892). 
(2) On the other hand, it was suggested that these discordant results as to 
the differentiation of the early cleavage cells might be explained by the fact that 
the eggs of different animals might differ in the time at which differentiations 
arise. In the eggs of echinoderms, Amphioxus, fishes and frogs, which had been 
