S64 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



nucleus remains near this pole. First and second cleavages are meridional 

 and equal, the cleavage furrows gradually progressing from the basal pole 

 (Fig. 177, A). The two cleavage planes correspond, respectively, to the 

 stomodeal or esophageal and the tentacular plane of the adult. The third 

 cleavages are somewhat oblique in opposite directions on each side of the 

 esophageal plane, so that four slightly smaller macromeres lie somewhat 

 apical to four larger ones (Fig. 177, B). Each of the eight cells then gives 



B 



D 



Fig. 177, A-D. — Early cleavages of ctenophore egg. A, the first cleavage furrow passing 

 through egg, nuclei near basal pole; B, eight-cell stage; C, sixteen-cell stage; D, later stage on 

 larger scale (after Ziegler, 1898). 



rise to a micromere from its apical region (Fig. 177, C); these micromeres 

 divide, and other generations of micromeres form (Fig. 177, D), resulting 

 in four more or less separated groups of micromeres, corresponding to the 

 four quadrants. According to Spek's observations with dark-field illumi- 

 nation, the green ectoplasm of the Beroe egg aggregates about the basal 

 region before each of the first three cleavages, and the cleavage furrow be- 

 gins there. The green ectoplasm forms the head of the cleavage furrow and 

 a layer on each side as cleavage progresses apically and, following first and 

 second cleavages, is again uniformly distributed over the surfaces of the 

 cells. Following the third cleavage, however, the green ectoplasm is local- 



