274 



GENERAL EMBRYOLOGY 



FIG. 127. Cleavage of isolated blastomeres in the egg of the Mollusc, Patella. 

 From Wilson. A-D, X 167; G, H, X 242; others X 208. A. Normal eight-cell 

 stage, viewed from upper pole. Fourth cleavage in progress. B. Normal thirty- 

 two cell stage, from side. C. The so-called "ctenophore stage" (normal) viewed 

 from upper pole. The primary trochoblasts are ciliated. D. Normal trocho- 

 phore of thirty hours, from left side. Body wall in section, prototrochal cells in 

 surface view. E. Second cleavage of an isolated micromere of the first quartet 

 (one of eight cells). F. Entire quadrant products of first and second quartet 

 cells, being formed much as in the normal egg. G. Larva of twenty-four hours 

 from one of eight cells (micromere). From side, showing trochoblasts below, 

 apical cells above. H. Product of primary trochoblast isolated from sixteen- 

 cell stage. /. First division of isolated first quartet. J. Division of isolated 

 basal cell of eight-cell stage, showing typical arrangement of these four cells as 

 in- the normal group of thirty-two. K. Larva of twenty-four hours, developed 

 from group like /, showing two secondary trochoblasts and two feebly ciliated 

 cells (? pre-anal cells), m, primary mesoblast cell; s.g, shell gland. 



