STRUCTURE AND DEVELOPMENT OP LOXOSOMA. 287 
embryos and the vestibular epithelium. It is hence probably 
the case that in Pedicellina the brood pouch can be almost 
completely shut off from the rest of the vestibule, nutrient 
substances being secreted into it by its glandular epithelium, 
which is largely increased in area by the formation of folds. 
This is, probably, in the main a consequence of the large 
number of the embryos present at the same time, and of the 
great increase in size which they experience during their stay 
in the brood pouch. 
In the whole course of the development of L. Tethyse, the 
increase in size of the embryo is small compared with that of 
L. Leptoclini and of Pedicellina; so that whereas the 
ovum of L. Tethvae is much larger than that of L. Lepto- 
clini, this difference in size being still very obvious in 
comparing, for instance, fig. 29 with fig. 60, or fig. 38 with 
fig. 62 (representing corresponding stages of the two species 
magnified to the same extent), the embryo of L. Leptoclini 
early equals that of L. Tethyse in size, the free larva of 
the former species being considerably larger than that of the 
latter. 
It is worth noting that the marked growth of the embryos 
of L. Leptoclini and of Pedicellina cannot probably be 
entirely ascribed to the fact that they devour particles of solid 
food, since in L. Tethyse, whose embryo is quite early pro- 
vided with a mouth, the increase in size during develop- 
ment is comparatively small. 
Segmentation, Formation of the Germinal Layers, and Completion 
of the Alimentary Canal ; Mesoblast. 
L. Leptoclini. — The ovum (fig. 25), on escaping into the 
vestibule, segments completely, at first into two (fig. 26), and 
then into four (fig. 27). The latter figure was drawn in the 
living condition, and shows the vitelline membrane present at 
this and some of the later stages. I have not observed polar 
bodies, this being probably a result of my almost exclusive use 
of the section method. At the next stage represented (fig. 
28), the embryo is a solid mass consisting of about eight cells, 
