IX 



MOLLUSCA 



295 



and Iq^'^ also divide into equal daughters, and give rise, as in 

 the larva of Polygordius, to four lozenge -shaped groups of four 

 cells. These sixteen cells are termed the primary trochoblasts 

 (Fig. 227, lq2ii, Iq2i2, Iq^ai, iq222). 



The cells of the second quartette of micromeres divide as follows : 

 2qi divides into the tip cells 2qii, which alternate with the four groups 

 of primary trochoblasts, and into four lower cells, 2q^^. The cells 2q^ 

 divide into four larger upper cells, 2q^i, and four smaller lower cells, 

 2q^^, situated near the vegetative pole of the egg. The cells 2q^i are 

 situated nearly side by side 

 with 2qi2. 3B 



Of the cells of the third 

 quartette 3a and 3b divide 

 spirally like all the cells 

 we have so far mentioned, 

 giving rise to Sa^ and 3a^, 

 3bi and 3b^, respectively ; 

 but 3c and 3d divide by 

 spindles so directed as to 

 converge towards the median 

 plane of the emhryo and 

 make equal angles with it; 

 and this division constitutes 

 the first appearance of bi- 

 lateral symmetry in the egg 

 (Fig. 228), which has- now 

 assumed the form of a hollow 

 blastula. 



The residual macromere 

 3D glides upwards into the 

 blastocoele, remaining for 

 a time in connection with 

 the surface by a narrow 

 neck; it then divides into 

 an internal cell, 4D, which 

 will form part of the mid- 

 gut, and into a superficial 

 cell, 4d, which is the mother cell of the mesoderm. This 

 gliding upwards of 3D, which we can only attribute to altered 

 cytotaxis, is the first sign of the process of gastrulation (Fig. 229). 

 The other macromeres, 3A, 3B, and 30, then, a little afterwards, 

 follow the example of 3D and migrate inwards, also remaining for 

 a short time in connection with the surface by long necks. Then 

 they also divide; but, in the case of each of these, both their 

 daughters form part of the wall of the mid-gut. 



As development proceeds further, divisions take place amongst 

 the apical cells, in the rosette cells, and in the cells of the MoUuscan 

 cross. The cells of the Annelidan cross divide only once more. 



Fig. 228. — Stage in the development of the 

 embryo of Patella coerulea, showing the first 

 division of the third quartette and the beginning 

 of bilateral symmetry seen in the direction of 

 the spindles in 3c and 3d. Viewed from the 

 vegetative pole of the egg. The apparent small 

 size of the macromere 3D is due to its movement 

 inwards, which is the beginning of the process 

 of gastrulation. The cells of the third quartette 

 are ruled with vertical lines. Those of the 

 second quartette are dotted. The residual 

 macromeres are white. (After Wilson, some- 

 what altered.) 



