110 GASTKOPODA. 



the posterior of those two macromeres which are in contact with one 

 another mesially, divides into two cells, the smaller of which shifts 

 towards the centre of the egg. The other three macromeres also give 

 oft' such a small cell towards the centre, so that there are now four 

 small entoderm - cells (Fig. 40 H). The posterior macromere then 

 divides into two large cells of about equal size (//, mes) and the other 

 macromeres also divide (H, rut). In Neritina, a similar process takes 

 place, hut the size and position of the cells is somewhat different (Fig. 

 40 G, me-i and ent). In Crepidala and Umbrella also (Fig. 48 B, p. 

 120), one of the posterior macromeres gives rise to an entomere and 

 to a cell which divides into a right and a left half. These two last 

 cells are the primitive mesomeres and, according to their origin, 

 either already lie in the primary body-cavity (G) or else are pressed 

 into that cavity later. This latter is the case when, as in Planurbis 

 (Fig. 40 H), these cells (mes) at first form a continuous circle with 

 the large cells (ent). A cleavage-cavity is sometimes first developed 

 at this stage, by the partial separation of the layer of micromeres 

 from the macromeres, or else it forms still earlier, so that even before 

 the stage represented in Fig. 40 H, the embryo may exhibit the 

 form of a blastula with a wall much thickened at the vegetative pole, 

 in which case an invagination-gastrula restilts (Planorbis).* In the 

 first case, however, in spite of the fact that the formation of an 

 epibolic gastrula has already commenced (G) or has been actually 

 attained through the failure of the micromeres to rise up from the 

 macromeres, an invagination may also take place later owing to the 

 appearance of a rather large cleavage-cavity. In this latter case, 

 however, the germ-layers may also already have appeared as rudi- 

 ments. The macromeres next give off at the vegetative pole a few 

 small cells (G and H, mt) which, together with the former, repre- 

 sent the rudiment of the entoderm. The rudiments of the tlrree 

 germ-layers are now visible ; the ectoderm has arisen from the micro- 

 meres, the entodern. is represented by the macromeres and their last 

 derivatives, and, finally, the mesoderm is found in the form of two 

 cells (derived from one of the [posterior] macromeres). \ 



* [The cleavage-cavity seems to be very variable in the Gastropoda, and even 

 in those forms in which it is most conspicuous, it is found to vary at different 

 stages of cleavage. This variation is most noticeable in Limax, and Kofoid 

 (No. XIV) thinks that this cavity is connected with the excretory processes of 

 the lilastomeres. The cavity is most developed in those Gastropods in which 

 the gastrula is embolic and, during invagination, it becomes temporarily 

 obliterated, but re-appears later (Planorbis, Uabl, No. 90). — Ed.] 



+ [It will be seen that if the interpretations given on p. 107 of the relation 

 between the first and second cleavage-plaurs and the axis of the adult body 



