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EMBRYOLOGY 



which is not clear, is composed of a shell-like envelope, a 

 granulated body contained in it, and a lid. The shell lies 

 with its cavity toward the ventral side TFig. 100 F). It 

 consists of two parts and, owing to small comma-shaped 

 bodies embedded in its free edge, acquires a striated appear- 

 ance (Fig. 100 D, U). Its contents consist of four large 

 cells of nearly equal size, which lie close together, and are 

 granular. Finally, the lid, which corresponds to the ventral 

 part of the urn, consists in turn of four cells, which unite, at 

 the point where they all abut on one another, to form the 

 knob of the lid (Fig. 100 D to G, I). Within the urn van 

 Beneden sometimes obser.ved a ciliation, which he ascribed 

 to the granulated cells. 



Fig. 100.—^ to G, infnsorif orm embryos and their development— .4 to D, of Dicyema 

 typws ; EtoG, of DicyemelJa Wagnerii (after van Benbdkn, from Balfoub's Compara- 

 tive Embryology). A to C, stages of development ; D, embryo seen from the ventral 

 side; E, from the right side ; F, from the front ; G, the " urn " isolated; gr, granu- 

 lated cells contained in the urn ; I, its lid ; u, the shell, which forms the floor of 

 the urn; r, highly refractive bodies at the anterior end of the embryo. 



The origin of the infusoriform embryo, although at first 

 sight quite different from that of the vermiform embryo, 

 can perhaps be referred to this. It takes place in the axial 

 cell of the rhombogenous individuals, though not directly, 

 being introduced by a preparatory process (Whitman). 



Near the nucleus of the axial cell there arise two new 

 cells, the nuclei of which in all probability originate from the 

 nucleus of the axial cell. These two cells soon multiply, but 

 not so rapidly as in the formation of the vermiform embryos. 

 They never exceed eight in number, and often only a few 

 are present. Before these cells develop further they undergo 



