DEVELOPMENTAL HISTOEY OF THE MOLLTJSCA. 
21 
The condition of the alimentary tract in the embryo of Aplysia major as we now 
leave it is exceedingly interesting, and is never presented by the embryo of Aplysia 
minor, nor by other Nudibranchs studied by me. It is in the condition of a pair of 
freely communicating gastric chambers, or a double chamber which is connected by a 
pharynx with the exterior, but is entirely devoid of rectum or anus. It would be 
important to ascertain how these latter organs make their appearance. At the same 
time, if the figures of the development of Pisidium pusillum are referred to, it will be 
seen that at one time Pisidium is in a closely similar condition, having a perforate 
pharynx leading into a double gastric chamber, which is suspended in a large body- 
cavity, and though possessed of a so-named “ rectal peduncle ” due to the very earliest 
feature of the development, yet this peduncle is relatively very small, and does not open 
to the exterior. 
Development of Aplysia minoi :—PleurobrancMdium, sp. — We must now go back to 
the earliest stages of development, to compare them with those of the smaller species of 
Aplysia figured in Plates 7 & 8 of this memoir. 
Plate 7. fig. 1 represents a single ovum of A. minor in the condition exactly corre- 
sponding to fig. 5 of Plate 5 of A. major. 
Fig. 2 represents a condition further advanced, in that the colourless cleavage- 
products have extended round the two yellow spheres. It corresponds exactly to fig. 7 
of Plate 1 ; but we observe this difference between the two. In A. minor the yellow 
yelk-spheres are, each of them, beginning to show evidence of a central pellucid nucleus. 
Plate 7. fig. 3 brings us on to the stage corresponding with Plate 5. fig. 9 ; and now 
the differences are more obvious between the two species. In the present species the 
outermost cleavage-cells have “condensed,” if that expression is allowable, to form a very 
clearly marked epiblast ( ep ). Already this is thickened at the aboral pole, to form the 
basis of the shell-patch (slip). The two pioneer-cells of the mantle (mn) are prominent ; 
and within we have, as in A. major , the yellow residual yelk-spheres (ry), and a mass of 
undifferentiated cleavage-products (x). But the condition of the yellow masses is very 
different to that of those in the same stage of A. major : their outline is strongly marked ; 
they retain their circular contour, and possess each a large brilliant and colourless 
nucleus. There is no question in this case of any breaking up of the yellow masses, or 
of their possibly furnishing formative elements by segregation to take the sole or a part 
of the work of building the hypoblast. They remain sharply defined, and keep their 
granular angular particles compacted together throughout the subsequent stages of 
development, although they become distorted and flattened by the pressure upon them 
of other growing elements, and probably dwindle and thin out in consequence of the 
absorption of some of their material. 
Plate 7. fig. 3 is very carefully rendered in every detail, as seen under a Hartnack’s 
10 d immersion. The figure represents an optical section in the median plane, and 
the region which will give rise to the foot is turned to the right. 
Plate 7. fig. 4 represents a similar view of an embryo a little further advanced, in 
