ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 93 



C. Cytoplasmic Localization. 

 1 . 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 1 874. 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 

 ai'ose 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 

 ami 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 T that there must be 

 some protoplasmic 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 on 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, " Selbstdifferenzi- 

 rni/o-" . of different blastomeres. 



Thus while the study of cell-lineaue 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 lie 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 lor any 

 specific end and that each might develop into any part of the embryo; if in any 

 ea-e certain parts or organs came from certain blastomeres it was due merely to the 

 '*' continuity of development" (Hertwig. ().. 1.892). 



(2) On the other hand, it was suggested that these discordant results as to 

 the differentiation of tin/ 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. Amphioxits.. fishes and frogs, which had been 



