x PODAXONIA 375 



intermediate cells of the Molluscan cross la 122 , lb 122 , lc 122 , also 

 acquire cilia and are incorporated in the prototrochal girdle. They 

 are the secondary trochoblasts (Fig. 303). Of the fourth quartette 

 of micromeres 4d is formed long before the others, and, as usual, 

 immediately divides into right and left sisters 4d r and 4d', which are 

 the mother cells of the mesoderm. The residual macromeres 

 eventually form a plate of endoderm which is situated beneath the 

 " b " arm of the Molluscau cross. 



As in Patella and Dentalium the process of gastrulation begins 

 by the sinking-in of this plate, and this in-sinking is caused by a 



end. 



<sf<m 



./ I \ I '^^^K'' 



Tnes 



FIG. 304. Nearly sagittal section of an embryo of Phascdosoma vulyare at the 

 stage when gastrulation is beginning. (After Geroulil.) 



a.p, apical plate ; il.c, dorsal cord ; end, endodermal cells derived from residual macromeres ; end 1 , 

 accessory endodermal cells, derived from the mesodermal band ; h.t>, head-blastema ; mes, tnesodermal 

 band ; p.tr, prototrochal cells ; st-om, cells which give rise to the stomodaeum ; t.b, cells forming the 

 trunk -blastema. 



change of shape in its component cells. These elongate and become 

 flask-shaped, the bulb of each flask passes into the segmentation- 

 cavity or blastocoele whilst the neck remains for a time connected 

 with the surface (Fig. 304). 



Further divisions at the upper pole of the egg result in the pro- 

 duction of a diamond-shaped apical plate of cells surrounding four 

 central cells, the apical cells, which acquire long stiff hairs and form 

 the apical sense-organ. The cells surrounding this apical plate then 

 begin to sink inwards so as to form a ring-shaped invagination which 

 is, as a matter of fact, the head-blastema. The trunk-blastema lies 

 on the ventral surface, behind where the blastopore is situated and 



