The Development of Ischnochiton. 617 



blasts as shown in Fig. 24 the four macromeres lie at the vegetative 

 pole. The cell D is, as has been explained, usually very small 

 comparatively, and as a result it plays a very inconspicuous role during 

 invagination. The three remaining macromeres on the other hand are 

 of considerable size extending within the embryo to a point perhaps 

 level with the prototroch. 



In the formation of the fourth quartette the macromeres in 

 quadrants Ä, B and C divide externally into almost equal products, 

 but following the cleavage furrow into the interior of the egg it is 

 found that the fourth quartette cells become thinner and wedge-like 

 and that the macromeres are consequently thickened and pear-shaped. 

 A general idea of the shape of these cells may be gained from Fig. 37 : 

 their inner ends are at about the same level. 



The mesoblast is a larger cell than any of the others mentioned, 

 the largest in fact on the vegetative pole of the egg, and further than 

 this needs no mention more than to state that the inner end of this 

 cell is about on the same level as the inner ends of the macromeres 

 of the other quadrants. 



The stomatoblast cells of the second quartette (2 a ^•■2. -2 etc.) are 

 at this period almost wholly without the region of the blastopore. 

 They will be more fully discussed later on when they come to take 

 part in the formation of the stomodaeum. 



The third quartette stomatoblasts are much more important cells. 

 In size they are superficially larger than any of the cells on the 

 vegetative pole except the mesoblast, and superficially they appear of 

 about the same size, but a careful study of stained preparations and 

 wax reconstructions at this stage make it clearly apparent that in 

 reality there is a considerable disparity in the masses of these cells. 

 The two anterior ones, roughly speaking, are ellipsoids, extending 

 therefore but a short distance into the egg being wedged in position 

 between the cells of the third and fourth quartette and the macro- 

 meres. For example, not infrequently 31) i-^ and 3h^-^ are in contact 

 with 4h^ B, 4a and Ä a little distance within the embryo, and the 

 stomatoblast thus occupies a trough between the cells. Not so the 

 posterior cells; lying closely appressed against the mesoblast they 

 extend inward often nearly to the same distance and the remainder of 

 the third quartette therefore is always separated from the mesoblast. 

 This latter with its two companion stomatoblasts constitutes a system 

 that is but slowly affected by invagination, upon which probably 

 depends to a large degree the shifting of the blastopore. 



40* 



