CLEAVAGE AND DEVELOPMENTAL PATTERN 545 



"spiral" because it may be conceived as resulting from a spiral twisting of 

 planes of radial or bilateral symmetry into surfaces forming a spiral about 

 the polar egg axis. There is, however, no evidence of such twisting, but 

 the term "spiral cleavage" has been so generally employed that it is used 

 here. In general, this pattern of cleavage gives way sooner or later to 

 more or less bilateral patterns; but as long as it persists, each spindle is 

 approximately perpendicular to that of the preceding division of the cell 

 concerned, and consequently successive cleavages are alternately dexio- 

 tropic and leiotropic. Because this form of cleavage occurs in the earlier 

 stages of polyclad turbellaria, annelids, gephyreans, and mollusks, except 

 cephalopods, and because it is sufhciently determinate in character to 

 permit the following of cell Hneage from first cleavage to certain regions 

 or organs of the larva or later embryos, it has received much attention and 

 has been largely responsible for the concept of ontogeny as a mosaic and 

 for theories of the phylogenetic significance of cleavage and of cell homolo- 

 gies.^ 



Various forms and stages of spiral cleavage are shown in Figures 171-76. 

 Different terminologies for designating the blastomeres have been used, 

 but the following has become more or less standard. The first cleavage is 

 meridional and may be equal (Fig. 171, A) or unequal (Figs. 172, A; 

 175,5). When it is unequal, the smaller cell is yl 5, the larger, CZ). Second 

 cleavage is also meridional and divides AB equally into A and B; and in 

 those forms in which the first cleavage is distinctly unequal, the second 

 divides CD into a smaller cell, C, and a larger, D. In polyclads and nemer- 

 teans the four quadrants are all alike and indistinguishable (Fig. 171, B). 

 In the third cleavage the first quartet of "micromeres" (la-id) separates 

 from four basal "macromeres" (lA-iD). The so-called "micromeres" are 

 usually smaller than the basal macromeres (Figs. 172, C; 173, A; 174, 

 A, C) but in some forms are equal in size to, or larger than, the latter 

 (Fig. 171, C); and definite size differences characteristic for the species 

 are often present, id and often ic being larger than la and ib (Figs. 172, 



' For more or less extensive descriptive studies of spiral cleavage see the following: Poly- 

 clads: Hallez, 1879; A. Lang, 1884; E. B. Wilson, 1898; Surface, 1907. Nemerleans: Zeleny, 

 1904. Annelids: Whitman, 1878; Salensky, 1882-83, 1885; Vejdovsky, 1883, 1892; Hatschek, 

 1886; E. B. Wilson, 1892; von Wistinghausen, 1893; Wheeler, 1897; Mead, 1897; Eisig, 1898; 

 Child, 1900; Treadwell, 1901; Schleip, 1914a; Penners, 1922, 1923, 1924a, b, 1925. Gephyreans: 

 Griffin, 1899; J. C. Torrey, 1903; Gerould, 1906. Mollusks: Rabl, 1879; Hatschek, 1881; 

 Blochmann, 1882, 1883; Heymons, 1893; Kofoid, 1895; F. R. Lillie, 1895; Meisenheimer, 

 1896, 1901; Conklin, 1897, 1907; Drew, 1899; Heath, 1899; S. J. Holmes, 1900; Robert, 1903; 

 E. B. Wilson, 1904; Wiersejski, 1906. 



