114 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



crescent and the ectodermal and endodermal areas. The chorda and neural plate 

 areas are also visibly different from surrounding areas at this stage (pp. 42, 50, 95, 

 97, 98, 108). 



23. In many eases the cleavage planes do not follow the lines of differentia- 

 tion but cut across them. Although cleavage is, under normal conditions, constant 

 in form, it is less constant and fundamental than the type of localization, and the 

 two are relatively independent (pp. 103, 104). 



24. The chief factor of localization is protoplasmic flowing; cell division is a 

 factor of subordinate value (pp. 102-104). 



25. Experiments which demonstrate the totipotence of blastomeres or regions 

 of the egg prove nothing with regard to the presence or absence of differentiation 

 in those parts. Some eggs with a high degreee of differentiation have at the same 

 time great capacity for regulation, e.g., those of ascidians ; 1 others with no greater 

 differentiation have little regulative capacity, e. g., ctenophores and mollusks. 

 Therefore the potency of any part of an egg or embryo is no satisfactory measure of 

 the degree of its differentiation (pp. 93-95). 



26. The organization of the ovocyte is not the initial organization. The yel- 

 low protoplasm (mesoplasm) of the Cynthia egg is probably derived, at least in part, 

 from sphere material (archoplasm) which arose from the nucleus at the last ovo- 

 gonic division. / The yolk (endoplasm) is formed by the activity of the "yolk 

 matrix" (Crampton) which also is probably sphere material. The clear protoplasm 

 (ectoplasm) is derived from the germinal vesicle at the first maturation division. Thus 

 many important regions of the egg come, at least in part, from the nucleus, and a 

 method is therein suggested .of harmonizing the facts of cytoplasmic localization 

 with the nuclear inheritance theory (pp. 99-101)./ 



27. There are several distinct types of germinal localization. /The annelid- 

 mollusk type does not approach that of chordates or echinoderms in the earliest 

 stages of localization more closely than in the cleavage or gastrular stages. There is 

 no convergence toward a common type in the earliest stages (p. 104-109). 



28. Embryonic repetitions (recapitulations), as well as many other homologies, 

 probably result from similarities of egg organization common to each type (p. 109). 



29. "Precocious segregation" is not a satisfactory explanation of the origin 

 of germinal organization (pp. 109, 110). 



/ 30. The evolution of animals must be accompanied by an evolution of the 

 type of germinal organization ; modifications of this organization are probably the 

 immediate causes of evolution. Transformations which would be impossible in 

 adults are readily brought about by modifications in the organization of the egg 

 {e.g., inverse symmetry). Perhaps profound mutations or even the origin of dis- 

 tinct types may be so explained (pp. 110, 111). 

 1 See foot-note p. 95. 



