286 
SIDNEY F. HARHER.- 
of the maternal tentacles, as is the case, according to Hatschek, 
in Pedicellina. In L. Tethyae there are no specialised 
brood pouches, and development takes place in the general 
cavity of the vestibule. In L. Leptoclini, however, where 
the amount of yolk is very small, the lining of the two brood 
pouches at the sides of the intestine consists of an epithelium 
of high columnar cells, altogether unlike the flat cells which 
line the greater part of the vestibule. During the earlier 
processes of development (figs. 25 — 34) the embryo hardly 
increases at all in size, if allowance is made for the appearance 
of internal cavities (blastocoel, archenteron), by which the cell 
layers are forced asunder. 
At about the stage of fig. 43, however, the epiblast in certain 
regions becomes very thin, and these portions are found closely 
applied to the columnar epithelium lining the brood pouches 
(fig. 47). The embryo forthwith commences to grow rapidly 
(figs. 43 — 45), and although this increase in size may be partly 
due to the digestion of food particles taken in by the mouth 
(which is already present), it is probable that it is mainly due 
to the activity of the columnar cells of the brood pouch. 
These may be supposed to secrete a nutrient material through 
the thin walls of the embryo into its primary body cavity, 
which at this stage is a wide space between epiblast and hypo- 
blast. The connection between the thin walls of the embryo 
and the epithelium of the brood pouch is often so intimate that 
careful observation is necessary to discover the existence of the 
former (see fig. 47). It is probable, therefore, that in L. Lepto- 
clini the embryos are nourished by a placenta composed of a 
layer of glandular cells, modified epidermic elements of the 
vestibule, forming a portion of the outer limit of the gela- 
tinous substance (of the adult), which serves as a medium for 
the diffusion of the various products of metabolism to the 
different organs. 
In Pedicellina echinata the epithelium lining the brood 
pouch is similar to that found in the corresponding position in 
L. Leptoclini. It is, however, thrown into numerous folds 
(fig. 12), whilst no intimate connection is set up between the 
