42 Moll 
MOLLUSCA. 
now comes to a depression in the wall of the germinal vesicle, and at 
length separates from the tail ; the head splits longitudinally, each half 
being visible within the vesicle. There is only one aster associated with 
the segmentation nucleus, but a second afterwards appears. The pheno- 
mena of division are similar to those observed in Helix \ Platner (2). 
Embryology of Patella . — The mature ova (0T2 mm.) have a thick 
chorion radially striated. The micropyle is a large irregular opening, 
from the bottom of which arise two large polar globules ; one of these 
has a rounded extremity with an indistinct nucleus. Segmentation is on 
the usual molluscan type. Two unequal parts are first formed, both of 
which divide successively ; a third division at right angles to these pro- 
duces successively stages with five, six, seven, and eight spheres. 
The blastospbere has four large cells at the vegetative pole, which 
form the rudimentary endoderm, and at the beginning of gastru- 
lation grow inwards and assume a wedge-like form ; they then ex- 
pand into club-shaped cells which almost fill the segmentation cavity. 
Two cells become ciliated and form the apical plate, while the velar 
cilia are developed around the equator. A large “ endo-mesoderm ” 
cell now appears on each side of the endoderm, thus rendering 
the embryo bilaterally symmetrical. Each of these cells now divides, 
one moiety becoming the primitive mesoderm cell, the other forming a 
part of the lining of the mosenteron. The number of cells in the mouth of 
the blastopore now increases to eight, and it begins to move towards the 
future ventral surface, occupying the apex of a V-shaped furrow directed 
towards the velum. The cells which constituted the walls and floor of 
this V afterwards form the stomodseum. There is a lateral swelling on 
either side of the blastopore, and as it moves forward these unite behind 
it so as to form a median protuberance, the future foot. The velum 
increases by the formation of support cells, while the shell-gland has 
appeared even before the closure of the blastopore, and includes most of 
the dorsal surface behind the velum. On either side of the embryo cap 
is a cell with a bunch of long, straight hairs, perhaps sensory in function. 
The coelom appears as a space between the mesenteron and ectoderm ; 
the shell-gland becomes invaginated, and a nautiloid shell appears on 
the dorsal aspect of the embryo, the foot becomes more prominent, 
and the otocysts appear. A very large percentage of the embryos 
develop abnormally, so that the investigation could not be completed ; 
Patten (1). 
The development of Fissurella is described by Boutan, with illus- 
trations ; an abstract taken from a preliminary communication will be 
found in Zool. Bee. xxii, Moll. p. 39. 
The fertilized ovum of Doris has been watched during its passage 
down the oviduct ; it enters a large canal with branching tubes, lined 
with irregular nucleated cells; in this “ albuminous gland” it receives its 
first coat. The next portion is the “ shell gland,” and has smaller cells, 
these two constituting the “ opaque portion” (Hancock) of the albumen- 
gland. Next are convoluted tubes with a single layer of elongated 
granular cells, secreting the jelly which unites the eggs into a string. D. 
testudinaria has a prostate gland ; Bolot. 
