CLEAVAGE AND DEVELOPMENTAL PATTERN 553 



mosaic, that is, it proceeds autonomously in the varous self -differentiating 

 parts.^ 



However, as data on different species accumulated, it became increas- 

 ingly evident that supposed cell homologies were in many cases less exact 

 than had been supposed. The first cleavage is said to be almost trans- 

 verse to the median plane in some annehds {Nereis, Chaetopterus) and in 

 some mollusks (Crepidula, Umbrella), while in certain polyclads (Thy- 

 sanozooyi, Lepto plana), other annelids {Clepsine, Amphitrite, Arenicola), 

 and mollusks {Planorbis, Unio, Trochus) it is approximately 45° from the 

 median plane. These differences necessitate the assumption of at least 

 some differences in cellular localizations of different formative substances. 

 The first cleavage may be equal or unequal, apparently without definite 

 relation to the fates of the cells. There is apparently a relation between 

 spiral cleavage and the later asymmetry in gasteropods: spirals of cor- 

 responding cleavages are reversed in direction in certain sinsistral species. ^ 

 Conkhn (1903a, b) suggested that this might result from reversal of polar- 

 ity in eggs of these forms, but no evidence of reversal has been found. 

 Moreover, spiral cleavage is not accompanied by any general asymmetry 

 in polyclads, annelids, and pelecypods. 



The later cleavages of the first quartet and the fates of some of the 

 cells differ in different species. Certain descendants of the first quartet 

 become cells of the prototroch in some species; while in others corre- 

 sponding cells form, but there is no prototroch. The first somatoblast, 

 2d, may be of the same size as other cells of the second quartet or much 

 larger, and this difference in size is apparently without definite relation 

 to the final relative size of the parts developing from it or to the stage 

 at which they differentiate. Also, the later divisions of 2d, as far as they 

 have been followed, differ as regards directions and sizes of products in 

 almost every species studied. Such differences are well shown in earlier 

 stages of the somatic plate in Arenicola, the part surrounded by heavy 

 line in Figure 172, £ and F, and the ectodermal teloblasts and early germ 

 bands of another annelid, Clepsine, similarly indicated in Figure 173, 

 D and E. Some cells of the second quartet are said to give rise to both 

 ectoderm and mesoderm in certain species; but in others, apparently only 

 to ectoderm. 



Perhaps the cell 4d, the second somatoblast, which becomes mesoderm 



' See, e.g., E. B. Wilson, 1892, 1894, 1896, 1898; F. R. Lillie, 1895; Mead, 1897; Conklin, 

 1897, 1898, 1907; Heath, 1899. 



3 Crampton, 1894; S. J. Holmes, 1899, 1900; Wiersejski, 1906. 



