Nassa mutabilis. This form, the development of which has been very thoroughly worked out by Bobretzky (No. 242), will serve as an example of a marine Gasteropod with a large food-yolk. The segmentation has already been described, p. 102. It will be convenient to take up the development at a late stage of the segmentation. The embryo is then formed of a cap of small cells which may be spoken of as the blastoderm resting upon four large yolk cells of which one is considerably larger than the others (fig. 104 A). The small and the large cells are separated by a segmentation cavity. The general features at this stage are shewn in fig. 105 A, representing a longitudinal section through the largest yolk cell and a smaller yolk cell opposite to it. The blastoderm is for the most part one cell thick, but it will be noticed that, at the edge of the blastoderm adjoining the largest yolk cell, there are placed two cells underneath the edge of the blastoderm (_me_). _These cells are the commencement of the mesoblast._ In the later stages of development the blastoderm continues to grow over the yolk cells, and as it grows the three smaller yolk cells travel round the side of the largest yolk cell with it. As they do so they give rise to a layer of protoplasmic cells (fig. 105, _hy_) which form a thickened layer at the edge of the blastoderm and therefore round the lips of the blastopore. These cells form the hypoblast. The whole of the protoplasmic matter of the yolk cells is employed in the formation of the hypoblast. The rest of them remains as a mass of yolk. A longitudinal section of the embryo at a slightly later stage, when the blastopore has become quite narrowed, is represented in fig. 105 C. The greater part of the dorsal surface is not represented. [FIG. 105. LONGITUDINAL SECTION THROUGH THE EMBRYO OF NASSA MUTABILIS. (After Bobretzky.) A. Stage when the mesoblast is commencing to be formed. B. Stage when the yolk is half enclosed. The hypoblast is seen at the lips of the blastopore. C. Stage when the blastopore (_bp_) is nearly obliterated. D. The blastopore is closed. _ep._ epiblast; _me._ mesoblast; _hy._ hypoblast; _bp._ blastopore; _in._ intestine; _st._ stomach; f. foot; _sg._ shell-gland; _m._ mouth.] Two definite organs have already become established. One of these is a pit lined by thickened epiblast on the posterior and dorsal side (_sg_). This is the shell-gland. The other is the foot (_f_) which arises as a ventral prominence of thickened epiblast immediately behind the blastopore. The hypoblast forms a ring of columnar cells round the blastopore. On the posterior side its cells have bent over so as to form a narrow tube (_in_), the rudiment of the intestine.