II. I 



CELL-DIVISION 



for the third division is meridional and unequal in such a manner 

 that the next stage eight cells is composed of two opposite 

 pairs of small and two opposite pairs of large cells. 



The mesomeres in the sixteen-celled stage of Echinoids are 

 bilaterally arranged. 



In the Cephalopoda the egg is large-yolked, and segmentation 

 consequently meroblastic. After the first two meridional divisions 



nr: 



"Of. 



c 7 



F. 



FIG. 14. Three segmentation stages in the blastoderm of Sepia offi- 

 emails', the segmentation is of the bilateral type. 7, left; r, right ; I-V, 

 first to fifth cleavages. The top sides of the figures are anterior. (After 

 Vialleton, from Korschelt and Heider.) 



the bilateral disposition sets in, for the furrows of the third 

 phase are unequally inclined to the first furrow in two halves 

 the future anterior and posterior halves of the egg (Fig. 14). 



The egg of Ascaris megalocepliala also exhibits a bilateral 

 cleavage, but not on the plan just described. The first division 

 is equatorial. Then the animal cell divides meridionally, and, 

 as it will prove, transversely, the vegetative cell latitudinally. 



