702 PROCEEDINGS OF THE ACADEMY OF [Oct., 



stands Ischnochiton (Heath, 1899), in which the formation of 4cl is 

 delayed until the 72-cell stage. Yet, in spite of these modifications, 

 which are plainly of a coenogenetic nature, it is a remarkable fact, 

 as Heath (1898) has pointed out, that the definitive mesoblast pre- 

 serves its origin from 3D. 



V. — Outline of the Further Development. 



In order to make the histories of the quartettes more intelligible, it 

 is desirable to give a brief account of the later history of Dinophilus, 

 up to the time of hatching. My observations in regard to the main 

 points agree essentially with the account given by Schimkewitsch 

 (1895) for the White Sea form. 



After the segregation of the germ layers, i.e., at the 29-cell stage, a 

 cleavage cavity of considerable size has been formed, and this cleavage 

 cavity persists up to a stage of about 54 cells w^hen it has reached its 

 maximum size (text fig. Ill, A). The ovum is now roughly spherical 

 in outline. Very soon, however, the ectoderm cells which roof the 

 cleavage cavity spread out and flatten dowm, thus greatly reducing it, 

 and finally causing it to be obliterated completely (text fig. IV, 

 A and B). This flattening of the cap of ectoderm produces a pro- 

 nounced change in the contour of the embryo, which is now decidedly 

 elongated at right angles to the egg axis, i.e., in an antero-postcrior 

 direction, and this elongation becomes more pronounced as develop- 

 ment progresses. The ectodermal cap — that is, all of the ectoderm 

 exclusive of the 2d group — now moves over the entoderm in a forward 

 direction. The various stages of this movement are illustrated in 

 text fig. IV, A, B, C and D. In the embryo A, the Ijoundary of the 

 first quartette, marked by a dotted line, lies in a plane nearly parallel 

 to the long axis of the embryo; the apical rosette and the vegetal pole 

 are still nearly opposite one another. In the embryo B the forward 

 movement of the ectodermal cap can plainly be seen to have begun. 

 Embryos C, D and E show^ the further movements leading to the 

 closure of the blastopore, which brings the first quartette into a posi- 

 tion at the anterior end of the embryo, after having rotated through 

 an angle of about 80 degrees. That part of the ectoderm which for- 

 merly marked the am'mal pole, the apical rosette or its derivatives, is 

 now just dorsal to the anterior pole of the embryo, while the boundary 

 of the first quartette lies in a plane almost at right angles to the long 

 axis of the embryo. This forward movement of the ectoderm is 

 brought about by the mitotic activity of the cells of the 2d group 

 dorsal to X — X, viz., x', x^-x'*, x^-x'^ etc., and probably also of cells de- 



