574 LILLIE. [Vol. VIII. 



large cells is produced dorsally, which invaginates after gas tru- 

 lation to form the shell-gland (Figs. 5 and 6)} 



I have already described the origin of a'^\ it is from this 

 cell that the larval mesoblast arises. As this is a point of 

 some importance, and is open to doubt on a priori grounds, I 

 shall describe the process in some detail. a~ divides by an 

 obliquely equatorial plane into a'^- '• and a^- -■ or Y. V is 

 larger than a-- 1- and lies nearer to the vegetative pole. Y 

 next buds off a small cell j' to the right, which lies a little 

 anterior to, and to the left of M (Fig. 4). Shortly after it 

 buds off another small cell j- to the left. Again it divides 

 and buds off j/3 posteriorly. Kis then gradually overgrown, 

 and comes to lie in the segmentation cavity anterior to 

 the entomeres. In the stage in which six large dorsal cells of 

 the shell-gland are present, it divides sagittally into two equal 

 parts. At about the time of the invagination of the entoderm 

 each cell Yl and Yr divides into two parts, the dorsal product 

 of which is in each case somewhat the larger (Fig. 5). But 

 there is this distinction between the divisions of M and the 

 divisions of Y: the divisions of M are typically teloblastic; 

 those of Fare not (Figs. 5 and 6). The larval adductor muscle 

 and some of the " Strangzellen " are formed from products of 

 Y. These structures are purely larval, and take no part in the 

 formation of adult tissues. 



We have then in the formation and setting apart of this 

 cell Y for this particular function an exceedingly instructive 

 example of the precocious segregation of tissue elements. It 

 seems as if in no other way could the adductor muscle, so 

 important for the existence of the glochidium, be formed so 

 early. Some light is thus thrown on the significance of the 

 blastomeres of segmentation stages. It indicates that the 

 mosaic arrangement is a derived condition, and has been 

 acquired as the best means for the early separation of tissues. 



The same cell in Nereis is the left stomatoblast (Wilson), 

 which functions very differently. It may be interesting to 



1 It is interesting to note that in no other Mollusc has any cell been described 

 comparable to the "first somatoblast " of Wilson. In the Gasteropods d^ does not 

 differ from the other micromeres of the same generation. 



