24 LILLIE. [Vol. X. 



of oblique divisions ; on the contrary the micromeres of the 

 first generation continue for some time to divide obliquely and 

 the same holds true for other cells. But from now on there is 

 no difficulty in recognizing the fact that we are dealing with a 

 bilateral embryo, and little by little all parts of the embryo are 

 brought into relations of bilateral symmetry. 



There is no more striking instance of this than in the be- 

 havior of the larval mesoblast Y, which during these changes 

 has been more and more overgrown by the surrounding cells 

 (PI. V, Fig. 59). It has budded off two small cells j' and 

 y'^ (Pis. Ill, Fig. 39 and V, Fig. 59), and I am inclined to 

 think, though I am not certain of it, that a third small cell 

 is budded from Y, before the latter enters the segmentation 

 cavity. Fig. 61 is an optical frontal section of a stage with four 

 large dorsal cells, products of X ; the larval mesoblast is here seen 

 to lie almost entirely within the segmentation cavity. Fig. 62, an 

 actual transverse section of a still later stage, shows Y in the 

 process of equal bilateral division. Thus, though in origin 

 asymmetrical, the larval mesoblast comes, apparently by active 

 m,igration, to be placed symmetrically in later stages. In Fig. 62 

 the next important change in the history of the larval meso- 

 blast is shown. Each half is dividing. The resulting cells are 

 shown in section in Fig. 66 (PI. V). In some of my prepara- 

 tions I have seen spindles in the two cells of the larval meso- 

 blast, which would lead to the formation of extremely small 

 cells lying against the ventral wall of the embryo near the 

 region where the oral plate later appears. In yet others I 

 have seen these cells fully formed. I am not certain that this 

 always takes place, but when it does it always precedes the 

 division just mentioned. 



6. The First Somatoblast. 

 To return to the cells X, X, from which the shell-gland is 

 formed. Fig. 60 shows the right cell already divided into 

 fairly equal parts and the left in process of division. The 

 order of division of these two cells is invariably as figured ; 

 the two divisions are never synchronous, but I have not been 

 able to correlate this difference of time relations with any 



