ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 95 
experiments in the world could not have shown as satisfactorily as direct observation 
has done the remarkable cytoplasmic differentiations and localizations of this egg. 
It seems, therefore, that this apparent conflict between the results of observa- 
tion and of experiment on the early development of the egg, between the prospec- 
tive tendency and the prospective potency of its various parts, can be harmonized 
neither by the claim that differentiations do not exist in the early stages of devel- 
opment nor by the assumption that differentiations appear earlier in some cases 
than in others. 
(3) It seems rather that the true explanation of this discrepancy is the one 
originally suggested by Roux (1892, 1895), v;z., that there is a difference in the 
regenerative or regulative capacity of different ova and that in the experimental 
studies referred to we are dealing with zwdizrect development or regeneration, as con- 
trasted with dzrec/ or normal development. Just as some adult forms show little 
capacity for regeneration or regulation while others of equally complex differentia- 
tion show this power in a high degree, so it seems that the capacity for regulation 
shown by eggs is more or less independent of the degree of their differentiation. 
To all appearances the ascidian egg is more highly differentiated than those of mol- 
lusks or ctenophores, and yet the former has a much higher regulative capacity 
than the latter. // thts view of the relative independence of differentiation and 
regulation be correct the conflict between the results of cell-lineage and of expert- 
mental embryology disappears, for the prospective tendency or the actual differentia- 
tion of а blastomere and its prospective potency deal with two distinct things. 
2. Localization before Cleavage. 
The phenomena of germinal localization have heretofore been studied for the 
most part during the cleavage and subsequent periods of development; only within 
the last few years has this study been extended to the egg before cleavage. Never- 
theless the brilliant researches of Driesch, Lillie, Boveri, Fischel, Wilson and Carazzi 
in this field have already yielded most important results, and are full of promise for 
future work. In some cases this localization of different kinds of protoplasm or of 
organ-forming substances has been directly observed, in other cases it has been 
inferred from the results of experiment, but in many instances both observation and 
experiment lead to the conclusion that the morphogenetic processes begin before 
cleavage. The work of Lillie on Uzzo (1901) and Chetopterus (1902), and especially 
experiments of Fischel (1897, 1898, 1903) on the ctenophore egg, and of Wilson 
(1903), and Yatsu (1904) on the nemertine egg have shown that definite regions 
of the unsegmented egg give rise to definite organs or regions of the embryo. 
Apart from the early separation of protoplasm and yolk which occurs in many 
yolk-laden eggs, localization of visibly different kinds of protoplasm in the unseg- 
mented egg has been observed in relatively few cases. Among the earliest observa- 
does not include the whole of the right Ша. half. —— blastomeres produce rounded masses cf 
cells but have no power to give rise to muscle, chorda, neural plate or sense organs, if they do not contain 
those portions of the egg which normally give rise to these ida 
