I lO 



the externally placed cells difterentiating themselves off as an ectoderm, while the central 

 mass gives rise to the yolk-mass of later embryos, and probably to the coelomic spaces. 

 (IV) Immediately after the difterentiation of the ectoderm, the yolk is equally distributed 

 throughout the (Miil)rvo. It mnv disappears from the ectoderm, and in the later stages is 

 found entirely in the large yolk-mass which occupies the greater part of the cavity. 

 (V) The yolk-mass becomes constricted, in front of the middle fC. levinseni) or behind the 

 middle (C. gracilis). The anterior part projects backwards over the posterior part as an 

 umbrella-shajjed mass of yolk, specially noticeable in C. levinseni. There is practically no 

 sign of histological differentiation in the yolk-mass at any stage observed, although a cavity, 

 probably the future enteron, appears in the posterior part of the yolk (C. levinseni). 

 (VI) The ectoderm undergoes a considerable amount of histological differentiation. 



{a) The antero-dorsal portion becomes highly vacuolated, and at its front c-nd a 

 larval sense-organ is formed. In C. gracilis this is a sharply marked pear-shaped organ 

 (fig. 190, s.o.)^ which reaches the exterior at the base of a slight depression of the surface, 

 while in C. levinseni., the organ is connected with a more definite invagination of the anterior 

 ectoderm. It is not impos.sible that this sense-organ may represent the similar organ found 

 in Tornaria (cf. Morg.a.n, 94. p. 33, PI. Ill, fig. 19). The anterior vacuolated ectoderm 

 in C. levinseni contains numerous refractive bodies, while in C. gracilis nearly the whole 

 of the ectoderm contains scattered pigment-ma.sses (fig. 190) which are probably excretor)-, 

 since their occurrence between the vitelline membrane and the epidermis indicates that 

 they are discharged to the exterior. 



{ti) The anterior half or more of the ventral ectoderm, in C levinseni, is somewhat 

 thickened and forms a glandular mass, described above as the "ventral thickening". This 

 tissue stains in the .same way as the thickened glandular epidermis of the anterior side of 

 the adult proboscis, and it may probably be identified with this part of the adult animal. In 

 C. gracilis, the corresponding part of the embryo forms a conspicuous "ventral invagination", 

 which is probably due to a flexure ot the emliryo within its vitelline membrane. 



{c) At the posterior end of the embryo is a vertical .slit-like invagination, or 

 "posterior pit" in C. gracilis (figs. 188 — 190); while in C. levinseni (figs. 199, 207 — 209) 

 there appears to be a longer slit-like groove exending from the posterior end along part 

 of the ventral surface of the embryo. 



{d) The remainder of the ectoderm, in both species, consists of much elongated, 

 very narrow cells. Distinct evidence that these are ciliated was obtained in some cases 

 (fig. 210). In the older embryos of C. levinseni, the ectoderm becomes folded in a 

 complicated way. An early stage in this process is shewn in fig. 210, but in older embryos 

 the folds acquire a much greater degree of comple.\ity. 

 (VII) The body-cavity is represented by five coelomic spaces, corresponding with those of the 

 adult. The proboscis-cavity may have distinct indications of a somatic epithelium, but there 

 is no proof that a splanchnic layer is present. The second and third cavities, on the 

 contrary, are probably sacs with a complete epithelial investment. The origin of the 

 coelomic spaces has not been traced. 



