DEVELOPMENTAL HISTOEY OF THE MOLLUSCA. 
19 
find a parallel outside the class Mollusca) that in A. minor there is a quantity of 
granular nutritive yelk, which, though not forming part of the substance of the hypo- 
blastic corpuscles, nor yet enclosed within the alimentary cavity, remains in contact 
with the developing alimentary canal lying outside its cavity *, as is seen on a very 
much larger scale in Loligo. 
Plate 5. fig. 21 displays the shell-patch when seen from above. It has now grown 
to some thickness, as may also be remarked in fig. 20. The patch is in the fresh 
condition, and its constituent cell-elements are not discernible; but the important 
feature which it exhibits is the groove or slight invagination. It thus presents the 
most striking correspondence with the grooved shell-patch of the Lamellibranch Pisidium 
described in the preceding contribution. 
Plate 5. fig. 22 is a portion of the foot of such an embryo as fig. 20, on which a 
little fresh water has been allowed to act. This separates and brings into view the con- 
stituent cell-elements of the epiblast. 
Plate 6. figs. 23, 24. We now pass to a much more advanced embryo. The 
shell is well marked and shovel-shaped. It is in this phase that I found so many of the 
shells loose in the egg-capsules and packed one within the other, the embryos to which 
they belonged having become broken up, either by a normal process or owing to some 
injurious conditions. 
The embryos now become very difficult to examine. The slightest pressure is apt to 
cause them to fall out of the shell, and endosmotic action swells out the body-wall in 
the way seen in fig. 23. At the same time the velum being now well grown, they swim 
about with incessant activity. A slight pressure is sufficient to rupture the embryo 
and separate the foot and velum from the rest, as seen in fig. 35. Such fragments show 
well, however, the true form of the velum at this period. In figs. 23, 24 the focus is so 
arranged as to give a surface-view of the mass of the alimentary cavity. The strongly 
marked sulcus results from the original separation of the yellow yelk-masses. 
Plate 6. fig. 25, 26, 27, 28 are different views of the shells at this stage of growth. 
The narrower end has the brownish-yellow colour belonging to chitinous substance. 
Plate 6. fig. 29 is a somewhat more advanced embryo, the focus taking a plane below 
the surface of the wall of the alimentary cavity. The tract of the pharynx is now very 
sharply marked out, though at present it is only a plug of ingrown epiblast, and not a 
tubular body. In the velum-area a thickening of the epiblast is seen forming a distinct 
boss or lobe, which appears to be the commencement of the cephalic nerve-ganglion. 
The shell is not represented in fig. 29. 
Plate 6. fig. 30 represents the alimentary cavity of the same embryo, more superficially 
focused, so as to display the sulcus and the disposition of the yelk-granules. 
* March. 7th, 1875. — Therefore in the mesohlastic cavity. Such a position being occupied by a part of the 
endoderm or hypoblast, suggests a comparison with the development of Sagitta, where the mesohlastic cavity 
has been shown by Kowalevsky to be simply an outgrowth of the primitive endoderm, as in Echinoderms 
according to Mecznikow. 
