I09 



epithelial elements characteristic of the rest of the ectoderm, while a certain amount of substance 

 which is faintly stained (with borax carmine) may represent the glandular cells of the adult 

 proboscis. This substance occurs, in the present instance, mainly near the bases of the ectoderm- 

 cells, although in another embryo it occurs in larger quantity throughout the ventral invagination, 

 which has lost the vacuolated condition just noticed. 



Some of the cells on the right side of the orifice of the ventral invagination in fig. 192 

 project individually to the exterior. By comparison with other sections of the same embryo, it 

 appears probable that this is a stage in the evacuation of excretory products. Turning to the 

 body-cavities, fig. 192 shews the anterior cavity, with a distinct somatic epithelium, while in 

 fig. 193 are seen the second and third cavities. The collar-cavities {S. cr) are separated from 

 one another dorsally by a slight median thickening of the ectoderm, perhaps the commencement 

 of the central nervous system. In their ventral region they pass round the outer borders of 

 the posterior body-cavities, an arrangement which is also indicated in fig. 196. Both pairs of 

 coelomic spaces are complete sacs, surrounded on all sides by an epithelium. In fig. 194 the 

 collar-cavities have disappeared, while the posterior coelomic sacs seem to be continuous ventrally 

 with the yolk. 



It would undoubtedly be possible to obtain fuller information with regard to the early 

 development of Cephalodisciis from the material at present at my disposal; but I have refrained 

 from sacrificing more than a small quantity of the specimens in order to study this subject. 

 It is moreover highly probable that satisfactorily preserved material will soon be forthcoming, 

 as is indicated by the short statement that has recently been published by Andersson (03). 

 The examination of these heavily yolked eggs and embryos would undoubtedly be facilitated 

 by the employment of specimens preserved by suitable histological methods. The history of the 

 development, so far as I have been able to follow it, by means of the embryos described above, 

 together with the few earlier stages I have examined, may be summarised as follows. 



(Ij The &go is large (300 — 400 a. in greatest length) and is richly provided with yolk. It 

 becomes surrounded by a vitelline membrane on passing into the cavity of the coenoe- 

 cium, and it remains inside this membrane during the whole of the earlier stages of its 

 development. The embryo is probably hatched at the stage represented by figs. 188, 198 — 200 

 (cf. Andersson, 03). 

 (II) Segmentation is complete. Fig. 186 (C. gracilisj shews the first cleavage, one of the 

 blastomeres being apparently preparing for a second division. Masterman (98, 2, p. 514, 

 PI. \\ fig. 89) describes an embryo which, according to his statement, is a larva divided 

 into two segments by a more or less equatorial furrow. Although I do not wish to dispute 

 the accuracy of this statement, there is nothing in Masterman's account to forbid the 

 supposition that the figure might represent an &gg divided into two blastomeres. 



I have found one or two late segmentation stages, from which the only conclusions 



I can certainly draw are that the cleavage continues to be complete, and that it gives 



rise to a solid embryo. 



(Ill) The solid embry^o just indicated consists of numerous cells, a large number of which are 



internal. The formation of the inner layer appears to take place by a process of delamination, 



