I04 



The general structure of the embryo may be ilhistrated by figs. 207 — 210. Fig. 207 

 shews practically the whole e.xtent of the ventral thickening {v. f.). It consists of numerous 

 o-land-cells, cut transversely, and stained intensely with iron-haemato.\ylin, separated from one 

 another by epidermic cells which are practically unstained. Posterior to the ventral thickening 

 is a deep median groove which may be traced through a number of .sections until it reaches 

 the posterior end of the embryo (fig. 209). It is one of the characteristics of the older embryos 

 found within the coenoecium that the epidermis is a good deal folded (fig. 210). It is thus 

 not easy to decide whether the invagination shewn in fig. 207 is more than an unimportant 

 fold. It does not appear, in the corresponding position, in figs. 205, 206, which may however 

 belonor to a younger embryo. But in the embryos of C. gracilis^ a definite invagination 

 (fi^s. 1 89, 190, post. p.), which may be termed the "posterior pit", occurs at the posterior 

 end. The fold shewn in figs. 207 — 209 may provisionally be referred to under the same name. 



Fio-. 208 still cuts the ventral thickening, and shews on the right side a few of the 

 refractive bodies which have already been noticed. The central yolk-mass is dilated in front 

 and narrower behind, the posterior portion being surrounded by a space which appears to be 

 blastocoelic in nature. The body-cavity consists of the five divisions characteristic of the adult. 

 The collar-cavities {d. c.') lie e.xternally to the blastocoelic spaces just mentioned, that of the 

 left side being specially sharply marked, although both of them are (juite definitely separated 

 from the anterior body-cavity. The posterior body-cavities {d. c.'^) are seen on either side of the 

 posterior pit. 



It is not possible to decide from this section whether the proboscis-cavity possesses an 

 epithelium or not. A clear .space external to its cavity is due to the fact that the gland-cells 

 of the ventral thickening do not extend quite through the thickness of the epidermis. The 

 collar-cavities and the metasomatic cavities have indications of nuclei internally to the membrane 

 which forms their outer boundary. The left metasomatic cavity contains posteriorly a mass of 

 cells which has the appearance of being a definite organ. 



In fig. 209 the ventral thickening is disappearing, and the ectoderm of the anterior end 

 has the vacuolated appearance which is characteristic of the region containing the refractive 

 bodies. Examined with a higher power, these appear to be rod-like accumulations of pigment- 

 like granules. The posterior ]jart of the ectoderm is not vacuolated. The umbrella-like character 

 of the yolk-mass is very obvious. The anterior part is separated from the more axially placed 

 mass b)- a region in which yolk-spherules are absent, or are present in very small numbers. 

 This region is bisected, on the left side (and throughout, in other specimens favourable for the 

 examination of this point), by a distinct line which may probably be regarded as part of the 

 posterior boundary of the anterior body-cavity. The collar-cavities are distinct, while the posterior 

 body-cavities are separated from one another by an obvious mesentery. There are still indications 

 of a a blastocoelic space surrounding the yolk, and limited externally by the splanchnic membrane 

 of the second and third body-cavities. 



In fig. 210, the yolk-mass is continuous with the epidermis. This may be regarded as, 

 in all probability, representing the l)lasto]jore and at tlic same time the future anus. The axial 

 part of the )olk contains a cavity. This ajjpears to be the enteric cavity, although neither here 



