IX MOLLUSCA 335 



3a, 3b, and 3c, are now budded off from their respective macromeres. 

 Only after this has happened do the anterior cells of the second 

 quartette, viz. 2a and 2b, divide into 2ai and 2a2, 2bi and 2b^ respec- 

 tively ; whilst the somatoblast X buds off from its upper border a 

 small cell Xg. 



All the cells of the first quartette now undergo renewed cleavage, 

 so that we have eight circles of cells, viz. Iq^^^ Iq"^, Iq^^^ Iq^^^, lq^"i 

 Iq2i2, lq22i, and lq222_ j^ t,]-^ggg divisions the members of each circle 

 belonging to the D quadrant divide before their sisters. We have 

 thus in Dreissensia the same typical divisions of the cells of the first 

 quartette which are found in Patella and Polygordius; but 

 Meisenheimer does not refer to or figure any conspicuous cross-like 

 arrangement of any of these cells ; on the contrary, he seems to imply 

 that they continue to have a concentric arrangement. 



At the lower pole of the egg a single representative of the fourth 

 quartette is now given off. This is 4d, which Meisenheimer calls the 

 " second somatoblast " ; but it is of course homologous with the 

 mother cell of the mesoderm in both Polygordius and Patella. At first 

 the mother cell of the mesoderm, which we may designate as M, touches 

 the second somatoblast ; but the latter gives off a cell towards the 

 vegetative pole which Meisenheimer calls x^, and this, along with x^ 

 and Xj, completely separates X and M (Fig. 264, D). 



After a few more divisions in the cells of the first quartette, the 

 first unmistakable traces of bilateral symmetry make their appear- 

 ance by the division of both X and M into right and left halves. 

 Then from each half of X a small cell, x^, is budded off posteriorly, 

 and the arrangement of the derivatives of the first somatoblast is as 



shown below, viz. : „ „ 



X - aig - X ; 



Each half of M also buds off a small cell, and then, by repeated 

 transverse divisions, a longitudinal plate of large cells which is the 

 rudiment of the shell gland is developed out of the two halves of X. 



Following the stage which we have just described, the process of 

 gastrulation begins. The residual macromeres 4A, 4B, 40, and 4D 

 sink inwards towards the blastocoele. The small cells given off from 

 the mother cells of the mesoderm sink in with them and go to build up 

 the wall of the mid-gut. The mother cells should therefore be termed 

 mesendoderm, not true mesoderm ; they themselves lie posterior to 

 the lip of the blastopore, and are partly invaginated with the endoderm 

 in the process of gastrulation. By repeated division they give rise 

 later (just as in Paludina) to a loose mesenchymatous mesoderm, out of 

 which the connective tissue and muscles of the adult bivalve are formed. 



The invagination of the mid-gut cells proceeds at first very slowly, 

 because their progress is impeded by the much more rapid and 

 conspicuous invagination of the cells forming the shell gland. This 

 latter deep invagination lasts only a short time. Soon the cells 

 forming the shell gland are again everted and form, as in Patella, a 



