In the later stages of segmentation the epiblast cells differ from the hypoblast cells in the absence of pigment. The segmentation cavity, if developed, is small. A perfectly regular gastrula is formed (fig. 107 A and B), which is preceded by the embryo assuming a flattened form. The blastopore is at first wide, but gradually narrows, and finally assumes a slightly excentric position. _It becomes not the mouth, but the anus._ When the blastopore has become fairly narrow, mesoblast cells (B, _me._) appear around it, between the epiblast and hypoblast. Whether they are bilaterally arranged or no is not clear; and though coloured like the hypoblast, their actual development from this layer has not been followed. [FIG. 107. FOUR STAGES IN THE DEVELOPMENT OF PALUDINA VIVIPARA. (Copied from Bütschli.) _ep._ epiblast; _hy._ hypoblast; _me._ mesoblast; _bl._ blastopore; _an._ anus; _st._ stomodæum; _sh._ shell-gland; _V._ velum; _x._ primitive excretory organ.] The velum appears about the same time as the mesoblast, in the form of a double ring of ciliated cells at about the middle of the body (B and C, _V_). The mesoblast rapidly extends so as to occupy the whole space between the epiblast and hypoblast, and at the same time becomes divided into two layers (C). Shortly afterwards a space--the body cavity--appears between the two layers (D) which then attach themselves respectively to the epiblast and hypoblast, and constitute the somatic and splanchnic layers of mesoblast. The two layers remain connected by transverse strands. By a change in the relations of the various parts and especially by the growth of the posterior region of the body, the velum now occupies a position at the end of the body opposite the blastopore. Immediately behind it there appear two organs, one on the dorsal and one on the ventral side. That on the dorsal side (_sh_) is a deep pit--the shell-gland--which is continuous with a layer of columnar epiblast which ends near the anus. The other organ (_st_), situated on the ventral side, is a simple depression, and is the rudiment of the stomodæum. Between it and the dorsally placed anus is a slight prominence--the rudiment of the foot. On the two sides of the body, between the epiblast and hypoblast on a level with the shell-gland are placed two masses of excretory cells, the provisional kidneys (D, _x_). These are probably _not_ homologous with the provisional renal organ of Nassa and other marine Prosobranchiata. At a later period a ciliated cavity appears in them, which probably communicates with the exterior at the side of the throat. In the later stages the foot grows rapidly, and forms a very prominent mass between the mouth and the anus. An operculum is developed somewhat late in a shallow groove lined by thickened epiblast.