io6 



the somatic wall of which there are some indications of an excessively thin epithelial lining. 

 The dorsal ectoderm, which has taken, in the main, the haematoxyhn colour, still shews at its 

 base remains of orange-coloured yolk-spherules. It further contains a certain number of vacuolated 

 structures, stained with haematoxyhn, and similar to those which compose the organ .v. o. seen 

 in fig. 202. These structures may probably be regarded as gland-cells. The rest of the ectoderm 

 consists ot closely packed narrow cells. In fig. 204 the yolk-free region in the middle of fig. 203 

 has extended as an elHptic line through the yolk-mass, which is thus divided into a central and 

 a peripheral portion. The former surrounds a spHt-hke cavity, which may be regarded as the 

 commencement of the future aHmentary canaL The ectoderm no longer contains any gland-cells. 

 On one side of the section is a narrow space (ó.c.) lined by an obvious epithelium. This appears 

 to be one of the collar-cavities, although the series of sections is not suitable for distinguishing 

 between the second and third body-cavities in the more posterior parts of the embryo. It is, 

 however, probable that in fig. 205 the ventral mesentery is that of the third body-cavities, 

 while the septum between these and the collar-cavities is not visible. I have assured myself that 

 in some of the embryos adjacent body-cavities may appear continuous as the result of defective 

 preservation. It can hardly be doubted that in the embryo here represented the paired body- 

 cavities have a splanchnic lining as well as the more distinct somatic lining, but there is no 

 evidence of the existence of a splanchnic layer in the region of the anterior body-cavity. 



The last remains of the anterior yolk-mass are seen in fig. 206, a few sections beyond 

 which the posterior yolk-mass becomes connected with the epidermis by the divergence from one 

 another of the two laminae of the ventral mesentery. In the more posterior sections a dorsal 

 mesentery seems to be indicatecl, in addition to the ventral mesentery. 



The consideration of figs. 203 — 206 shews that in this embryo the anterior body-cavity 

 sends a median prolongation backwards, on the dorsal side of the yolk, between the halves of 

 the paired cavities. A similar arrangement is shewn in fig. 201, an obliquely longitudinal section, 

 in which although the three segments of the body-cavity are seen with moderate certainty on 

 the dorsal side (left of figure), it is not quite certain that the collar-cavity is represented on 

 the ventral side. 



(ó) Embryos of C. gracilis. 



The oldest embryos of this species have the form represented, in side view, in fig. 188. 

 The embryo here shewn measures 300 y.. in length, and like all the other embryos, both of this 

 species and of C. levinseni, it is still enclosed in the vitelline membrane. lts most conspicuous 

 organ is a large invagination of ectoderm [v. inv.), on what may be regarded as its ventral side. 

 At the a,nterior end is a median organ [s. o.) characterlsed by its transparency, while at the 

 opposite end is a median depression of the ectoderm {posf.p.). In the interior of the embryo is 

 a mass of yolk, which can be seen to be composed of two portions separated by an indentation. 

 The refractive bodies characteristic of the anterior end of the embryos of C. levinseni do not 

 occur, or at least not in precisely the same form. They may, however, be represented by cells 

 containing a brownish pigment which are scattered irregularly throughout most of the ectoderm, 

 though found in somewhat larger numbers near the posterior pit. 



