546 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



C; 173, A; 174, A). Division of both apical and basal quartets follows, 

 the apical usually preceding and giving rise to ia\ ia^-id\ and id\ the 

 basal to 2a-2d and 2A-2D. In most forms four or live quartets of micro- 

 meres are formed, each farther from the apical pole than the preceding, 

 before the spiral pattern is altered or obliterated in the cells concerned. 

 Since the spindles are alternately dexiotropic and leiotropic, the blasto- 

 meres interlock; and when the cleavages are equal or nearly equal, the 

 planes of contact are essentially similar to those resulting from surface 

 tension, as in a mass of soap bubbles (Robert, 1903). As far as known, 



Fig. 171, A-C. — Early cleavages of the nemertean Cerebratuhis (after Zeleny, 1904) 



certain descendants of the first quartet form the whole or part of the 

 prototroch; but numbers and origins of cells in prototrochs of different 

 species differ widely, and the series of divisions leading to its formation 

 also differ, or, when similar, are similar because they are spiral cleavages. 

 And in forms without prototroch, cells equivalent in origin to the trocho- 

 blasts form other parts of the ectoderm. Later divisions of the pretrochal 

 cells about the apical pole also differ in different species. The second 

 quartet of micromeres was regarded by Lang (1884) as entirely meso- 

 dermal in polyclads; but, according to E. B. Wilson (1898) and Surface 

 (1907), these cells in all four quadrants give rise to both ectoderm and 

 mesoderm. In the gasteropod Crepidula three of these cells {2a, 2b, 2c) 



