CLEAVAGE AND DEVELOPMENTAL PATTERN 551 



man, 1878; Schleip, 1914), which contain much yolk; in Dentalium eggs 

 they appear less definitely bounded and are separated superficially by a 

 pigmented, yolk-containing zone (E. B. Wilson, 1904). Some other eggs 

 show one or both of them in differing degrees. In consequence of the 

 definite character of cleavage these polar plasms are included in certain 

 cells; in Clepsine both are entirely or almost entirely included in the cell 

 CD by the first cleavage and in D by the second. Before the third cleavage 

 the basal polar plasm migrates apically and unites with the apical plasm, 

 and the entire mass remains in iD at the third cleavage and at the next 

 division is divided between 2d and 2D. The ectodermal germ bands de- 

 velop from descendants of 2d, and the mesoderm bands from 4d and 4D. 

 These derivatives of D constitute almost the whole mass of the cell, 3d 

 being small. Very similar behavior of the polar plasms has been described 

 for oligochetes, and eggs in which polar plasms do not appear fail to 

 develop or die in early stages and fail to develop somatoblasts (Vejdovsky, 

 1888; Penners, 19246). Penners apparently believes that failure to de- 

 velop and death result from absence of the polar plasms, but it seems 

 possible that absence of polar plasms may result from physiological or 

 pathological conditions and is only incidentally associated with develop- 

 mental failure and death. 



A temporary cytoplasmic yolk lobe or polar lobe appears basally in 

 many mollusks and annelids at the first cleavage and in some species 

 again at second and third cleavages. The three successive lobes of the 

 first three cleavages of Dentalium are shown in Figure 175, A-D. Nor- 

 mally the lobe may become almost completely separated from the rest of 

 the egg by constriction ; but it is actually a part of one blastomere — CD 

 in the first cleavage, D in the second, and iD in the third. In Dentalium 

 it consists largely of the unpigmented basal plasm, but in some other forms 

 it contains mostly yolk. However, in lobe-forming centrifuged eggs the 

 lobe appears quite independently of the cytoplasmic stratification and 

 may consequently differ in content in different individuals, according to 

 the direction of stratification (p. 585). Its formation in connection with 

 cell division and the flow of cytoplasmic substance into it as it enlarges 

 led Boveri (1910a) to suggest that it represents apart of the cell not in- 

 cluded in the sphere of influence of the adjoining aster; but the periodic 

 form changes in isolated lobes of Ilyanassa, corresponding more or less 

 closely to periods of the division cycle, occur in cytoplasm entirely isolated 

 from the dividing cell (Morgan, 1933, 1935, 1936). Hydrostatic pressure 

 of 220 atmospheres, applied in early stages of lobe formation, brings about 

 withdrawal of the lobe and inhibition of cytoplasmic division in Chae- 



