n6 



BIOLOGICAL LECTURES. 



ceases abruptly at the 64-cell stage. The cells of this stage 

 are, of course, all of the same generation, the seventh, counting 

 the ovum as one ; yet, as a consequence of the obliquity of the 



cleavage furrows, the cells are 

 not directly superimposed, but 

 alternate with one another. 



For many reasons it is help- 

 ful to conceive of the egg as 

 consisting of four quadrants, 

 each of which is composed of 

 the derivatives of one of the 

 cells of the 4-cell stage. Any 

 cell in one quadrant corre- 

 sponds in origin and in posi- 

 tion to a cell in each of the 

 other quadrants. 



FIG. 5. A mphitrite, left side, four cells getting Let US Consider the Cgg in 



ready to divide into eight. 



the 4-cell stage (Fig. 5) as 



composed of one quartette of cells about to divide. Four of the 

 descendants of each of these cells will belong to the primary 

 prototroch. In a similar way the egg in the 8-cell stage con- 

 sists of two quartettes of cells, 

 those of one quadrant meet- 

 ing at the anterior pole, the 

 position of which is indicated 

 in Fig. 6 by the polar glob- 

 ules, those of the other meet- 

 ing at the posterior pole. T/ie 

 primary prototrochal cells are 

 all descended from the cells 

 of the anterior quartette, and 

 from that portion of the cells 

 which is stippled in Fig. 6. 

 The cells of this quartette, or, 

 in the latter stages their descendants, constitute the anterior 

 hemisphere, or umbrella, and. the cells of the posterior quartette 

 constitute the posterior hemisphere, or suburnbrella. The hemi- 

 spheres are separated in the drawings by a dotted line. 



FIG. 6. Amphitrite, left side, last phase 

 of 8-cell stage. 



