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 eases 

 than in others. 



(3) It seems rather that the true explanation of this discrepancy is the one 

 originally suggested by Roux (1892, 1895), viz., 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 indirect development or regeneration, as con- 

 trasted with direct or normal development . Just as some adult forms show little 

 capacity tor 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. If this view of the relative independence of differentiation and 

 regulation be correct the conflict between the results of cell-lineage and of experi- 

 mental e/ubryology disappears, for the prospective tendency or the actual differentia- 

 tion of a blastomere and its prospective potency deal with two distinct things. 



2. Localisation 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 Unio (1901) and Chestopterus (1902), and especially 

 experiments of Fischel (1897, L898, 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 }*olk 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- 



dnes not include the whole of the right or lilt half. Individual blastomeres produce rounded masses cf 

 cells but have <> power '" give rise to muscle, chorda, neural plate or senst organs, if they do nut contain 

 thost portions of the egg which normally givt rise to these parts. 



