334 



INVEETEBEATA 



CHAP. 



quartette divide again, so that we get four concentric circles of cells, 

 Iq 11 , lq 12 , lq 21 , and lq 22 . 



The third quartette of micromeres now begins to be formed, 3d 

 being formed before its sisters. X gives rise to a small cell on the left, 

 the proper title of which is 2d 12 , but which is called by Meisenheiuier 

 x 2 . The somatoblast has thus acquired at its lower border a wreath of 

 three cells, x p 3d, and x 2 . Of the second quartette of micromeres, which 

 should have divided when the third quartette was being formed, only 



M4D 



V 4A 



FIG. 264. Stages in the cleavage of the egg of Dreisse'iisia polymorpha. 

 (After Meisenheimer. ) 



A, upper hemisphere of egg in the 16-cell stage. B, upper hemisphere of egg just passing into the 

 54-cell stage. The formation of the apical cells is seen. C, egg seen from the vegetative pole in the 

 8-cell stage at the moment when 2d is being formed. D, Posterior view of egg in a somewhat later 

 stage than that shown in B, to show the primary mesoderm cells and some of the products of the 

 division of X. M, primary mesoderm cell ; jj.b, polar body. 



one (2d = x) has as yet divided. Another member of this 

 quartette now divides, i.e. 2c ; whilst 2d 2 divides into 2d 21 and 2d 22 

 or, according to another notation, x l gives x u and x 12 . 



It is quite clear therefore that in Dreissensia, unlike Patella, the 

 radial symmetry of the spiral type of cleavage is very early interfered 

 with, and that the prospective importance of the organs derived from 

 2d is reflected back into a very early stage of ontogeny; this is testified 

 to by the precocious divisions and development of the cells derived 

 from this blastomere. 



The remaining members of the third quartette of ectoderm cells, 



