IX 



MOLLUSCA 



293 



Now, as a result of the study of eggs with spiral cleavage belonging 

 to many different species, it has been found that in all cases of normal 

 cleavage i.e. where D is the left posterior macromere the cross 

 furrow at the vegetable pole slants upwards from left to right when 

 looked at from above, the inacromeres being placed in their normal 

 position. But we can determine what is their normal position from 

 the fact that the polar bodies are given off from the animal pole, and 

 if we then look at the egg from this pole and rotate it until the cross 

 furrow takes up the proper position, we know that the left anterior 

 blastomere is A, the right anterior B, and so on (cf. Fig. 226). The 

 cross furrow does not always 



appear in the early stages in io\ _ i / P 



v 4 u j T_ -.L j - ' V^ ^r>^^^lo 2 



Patet/a, and when it does not, 



it is impossible to be sure of 

 the orientation of the egg. 



The 8 -cell stage is reached 

 as usual by the fourth cleavage, 

 and in it the four upper 

 cells or the first quartette 

 of micromeres are decidedly 

 smaller than their lower sisters 

 the niacrorneres (Fig. 226). The 

 micromeres are separated from 

 the inacromeres as usual by 

 the formation of dexiotropic 

 spindles. 



At the next cleavage the 

 first-formed micromeres divide 

 by laeotropic spindles into a 

 set of cells, Iq 1 , above, and a 

 set, Iq 2 , below. The cells de- 

 noted by the symbol Iq 2 are 



slightly larger than those named Iq 1 . At the same time the inacro- 

 nieres also divide laeotropically, giving rise above to . the second 

 quartette of micromeres. These micromeres are larger than the first 

 quartette, but still much smaller than the residual rnacromeres 2 A, 

 2B, 20, and 2D. 



At the next cleavage the 32-cell stage is attained. Each of the 

 basal cells 2A, 2B, 20, and 2D divides dexiotropically so as to give 

 rise above to a small daughter cell. These four smaller cells 

 constitute the third quartette of micromeres, and, as in Annelida, 

 the three quartettes give rise to all the ectoderm. The second 

 quartette cells divide, at the same time, each into two almost equal 

 daughter cells. The lower daughter cells derived from the first 

 quartette Iq 2 divide quite equally into upper cells Iq 21 and lower Iq 22 . 

 As in the eggs of Polygordius, all the descendants of Iq 2 enter into the 

 formation of an equatorial band of ciliated cells or prototrochal 

 girdle, which later encircles the larva about its equator. The 



FIG. 226. Development of Patella coerulea. 

 8-cell .stage showing the spindles preparatory 

 to the formation of the 16-cell stage. The 

 spindles in the upper cells show the laeo- 

 tropic twist. (After Wilson.) 



i'.f, cross furrow at the vegetable polfl ; pJ>, polar 

 bodies. 



