I07 



Other embryos have a more spherical form, as for instance that shewn in fig. 187, a 

 view iVom tlie ventral and anterior sides. The anterior transparent organ {s. o.) is indicated 

 externally by a depression, while the orifice of the ventral invagination is very conspicuous. 



Fig. 1 89 represents a spherical embryo seen from the ventral side, some of the organs 

 being shewn in optical section. It is probably somewhat older than fig. 187, the aperture of the 

 ventral invagination being considerably shorter from before backwards than in that specimen. An 

 emargination occurs in the outline at each end of the sagittal plane of the embryo, representing 

 respectively the transparent organ {s. o.) and the posterior pit [post.p). These emarginations 

 are even more conspicuous in the younger stages. 



The most striking difterence between these embryos and those of C. Icvinseni is due to 

 the presence of the large ventral invagination in C. gracilis. It seems to me probable that this 

 is the representative of the ventral thickening of the embryos of C. levinseni, and that it is to 

 be regarded as a ventral tiexure, comparable with that of the embryos of other animals which 

 undergo part of their development within an egg-membrane. It ma\- be supposed that with the 

 escape of the embryo from the vitelline membrane, the ventral invagination will flatten itself 

 out and the embrvo will therebv assume a more elongated form. 



Fig. 197 is a sagittal, nearly median section of an advanced embryo of C. gracilis, 

 shewing the transparent organ (or. <?.), the ventral invagination [v. inv.) and the edge of the 

 posterior pit ipost.p.). The internal )-olk-mass has an emargination dorsally, in which lies a well 

 marked coelomic vesicle [b. er). It can hardly be doubted that this represents the collar-cavity, 

 and accordingh' that the undivided space [b. c}) in front of it is the anterior body-cavity, while 

 the spaces [b. c?) at the opposite end are the third body-cavities. Both the right and the left 

 cavity, with the ventral mesentery, are here seen, in consequence of a slight obliquity of the 

 section. The anterior third of the ectoderm, between the two arrows shewn in the figure, is 

 vacuolated in this embr\o, while the remainder of the ectoderm is composed of narrow, closelv 

 packed epithelial cells of considerable height. 



Figs. 190, 196, 195 are three horizontal sections of a similar embryo. Fig. 190, which 

 shews some histological detail, passes through the dorsal end of the ventral invagination {z'. inv.) 

 the epithelium of which is cut tangentially ; and the lumen of the organ is thus not seen. 

 Rather more than half of the ectoderm, at the anterior end, has a vacuolated structure, 

 resembling that of the same part in C. Icvinseni-. The rest of the ectoderm is composed of 

 narrow, closely packed cells. In all parts of the ectoderm, except in the anterior transparent 

 organ, occur the pigmented cells noticed in the entire embryos. There is some indication, in 

 this and in other cases, that an accumulation ot the pigment occurs on either side of the 

 organ .s-. o. There is distinct evidence that rounded masses of the pigment are passed to the 

 outer side of the ectoderm, where they are found within the vitelline membrane (fig. 197, exc), 

 an observation easily made both in sections and in entire embryos. It ma)- naturally be suggested 

 that this embryonic pigment is of excretory nature; a conclusion which perhaps carries with it 

 the consequence that the refractive bodies found in the embryos of C. Icvinseni., and moreover 

 the epidermic pigment of the adult Cephalodiscus, ma)- have the same physiological value. 



The transparent organ [s. o.) has a pear-shaped outline, and consists of a highl)- vacuolated 



