CLEAVAGE AND DEVELOPMENTAL PATTERN 563 



cleavage of rhabdocoels and triclads is apparently not spiral; also, trem- 

 atodes and cestodes seem not to have spiral cleavage. Rotifer cleavage 

 pattern is apparently spiral in early stages but later departs from spiral 

 pattern (Zelinka, 1891; Jennings, 1896). If the rotifers are related to an- 

 cestral forms of the "Trochelminthes" and if the spiral cleavage pattern 

 has ancestral significance, we might expect to find it in well-developed 

 form in the rotifers. Cleavage in cephalopods is meroblastic and widely 

 different from the spiral type (Vialleton, 1888; Watase, 1891). Spiral 

 cleavage may appear to be closely associated with early differentiation of 

 free-swimming larvae of trochophore type, but in oligochetes and leeches 

 and in fresh-water pelecypods there are no such larvae, but cleavage is 

 spiral. Also, the piHdium larva of nemerteans is very different from the 

 trochophore type, and some nemerteans develop without pihdium larvae; 

 but spiral cleavage is present in both.'" 



Whatever conclusions we may draw concerning the phylogeny of groups 

 with spiral cleavage and their relatives without, it seems probable that we 

 must distinguish between the obhque, or so-called "spiral," type of cleav- 

 age, as determined by physiological factors not closely associated with or- 

 ganismic developmental pattern, and the relative sizes of cells and their 

 differences in different groups, as expressions of that pattern. The obhque 

 form of cleavage corresponds closely to a surface-tension pattern, but rela- 

 tive cell sizes suggest regional differences in cortical activity or differences 

 in the deeper cytoplasm. 



CLEAVAGE AND RECONSTITUTION IN CTENOPHORE DEVELOPMENT 



Pattern of cleavage and early development of ctenophores differs from 

 those of other groups, and present knowledge serves chiefly to make evi- 

 dent the lack of any real insight into its physiology." In the undivided 

 egg of Plenrohrachia a thick ectoplasm or cortical layer, uniformly dis- 

 tributed about the egg, stains yellow with neutral red and rose with Nile 

 blue sulphate, as if alkaline in reaction, in sharp distinction from the in- 

 terior, which stains red or blue. The egg of Beroe shows no such differen- 

 tial staining, but with dark-field illumination a clearly defined green ecto- 

 plasm or cortex is visible (Spek, 1926). Polar bodies form at what is con- 

 sidered to be the basal or vegetal pole, that is, the later oral pole; and the 



" Spiral cleavage has been followed through the earlier stages in a fresh-water nemertean 

 Stichostemma, with direct development (Child, unpublished). 



" Chun, 1880, 1892; Driesch und Morgan, 1895; Fischel, 1897, 1898, 1903; Ziegler, 1898, 

 1903; Rhumbler, 1899; Yatsu, 1911, 1912a, 6; Spek, 1926. 



