IG 



EMBRYOLOGY 



of eiglit cells, which enclose the cleavage cavity. Since the cells 

 are applied to the wall of the tube with their broad bases, 

 and taper conically in the opposite dii^ection, the embryo has 

 nearly the form of a cup-cake (Fig. 2 D). By means of a 

 subsequently appearing equatorial furrow each of these 

 8 cells is separated into an upper smaller [entodermal] and 

 a lower larger [ectodermal] segment ; and at the same time the 

 whole shape of the embryo changes in this 16-cell stage, 

 taking on the form of a biconvex lens by the bulging out of 

 its basal surface (Fig. 2 E and F). The cleavage cavity is 

 still open at both poles, although the opening of the upper 



ec 



c.s. 



Pig. 3.— Swarming larval stage of Sycandva raphanua (after F. E. ScnuLZK, from 

 Balfour's Comparative Embryology). A, amphiblastula-stage; B, stage at the 

 beginning of the gastrula invagination; cs, cleavase cavity; ec, future ectoderm 

 colls ; en, future entoderm cells. [The orientation of the larva in this figure is the 

 reverse of that in Pig. 2 D, Pand If, the entodermal pole being there the upper 

 one.]— Tbanslatobs. 



side is already considerably narrower than that of the lower 

 one. By means of new meridional and equatorial furrows 

 the embryo gradually passes into a multicellular stage, 

 which has an almost spherical shape, corresponding to which 

 there is an extensive cleavage cavity within. The opening 

 at the upper pole has disappeared by the apposition of the 

 cells, while the one corresponding to the former basal sur- 

 face is still retained (Fig. 2 G). It is surrounded by eight 

 [ectodermal] cells, which are soon distinguished by increasing 



