DEVELOPMENTAL HISTORY OF THE MOLLUSCA. 5 
section. In fig. 22 the hypoblast is in section, showing clearly the character of its 
cells and their arrangement. In fig. 23 its somewhat irregular outgrowths (hy) are 
seen, and their relation to groups of the branched mesoblastic corpuscles. 
The most important fact shown by these two drawings is the arrangement of certain 
of the mesoblastic branched corpuscles (me) in strings or groups, binding, as it were, 
others of the cells into groups. The large cells (2) of these figures are not distinguish- 
able in form and character from the epiblastic cells in immediate contact with which 
they lie, and from which, with little doubt, they have been derived. They apparently 
furnish the primitive elements of the foot; but whether they are to be considered 
distinctly as mesoblastic elements or as epiblastic I cannot decide. Supposing that 
they give rise to muscular tissue, they establish a very close connexion between the 
* Hautfaserblatt” and the epiblast, which is paralleled in Hydra and in the higher 
Ceelenterata. On the other hand, it is possible that the strings of branched corpuscles 
(derived at an earlier period from the epiblast) which traverse these groups of large 
cells are the real foundations of the muscular tissue, and that the large cells serve only 
as so much material for their appropriation, or as the primitive elements of the nerve- 
ganglia. 
Plate 2. fig. 24 is not so far advanced in development as figs. 22, 23. It presents two 
features of interest with regard to the mesoblast. First, several large fusiform 
corpuscles of the mesoblast are seen attached by one extremity to the mass of the 
hypoblast, and by the other connected with groups of mesoblastic cells. In connexion 
with a similar condition in the corresponding stage of development of Aplysia, to be 
described in a further communication, this has considerable interest; and so has the 
second feature, which also is presented by Aplysia, viz. the ciliation of the surface of 
some of these mesoblastic cells (ci). In this particular embryo a few cilia were seen 
also on the outer surface of the epiblast, as indicated in the figure. 
The ciliation of the mesoblastic cavity is a common phenomenon in adult Vermes 
(Gephyrea and some Annelids), and has even been observed in a sporadic form in some 
Vertebrata exclusive of the ciliation of the Fallopian tube (Klein’s observations on the 
peritoneum of the Frog). 
Plate 2. figs. 25, 26 bring us to a later stage and more definite differentiation of 
parts than we have yet considered. They represent the same embryo, seen first in 
section and then from the surface. The rudimentary alimentary cavity (a/) is seen 
hanging from its rectal peduncle as in fig. 20. It will hardly be right any further 
to speak of this mass as simply “hypoblast;”’ for it is by no means clear what 
changes haye gone on, and there may be elements now present which represent the 
“ Darmfaserblatt.” The rudimentary alimentary sac is seen to have definite lobes now 
which will shortly develop into the two large juxtaposed chambers which constitute the 
bulk of the alimentary canal during the embryonic condition in Pisidium. ‘The epiblast 
(ep) is seen not to be sharply marked off from a number of cells or corpuscles accumu- 
lated beneath its wall, both laterally (me) and at that pole which represents the foot (/). 
