DISTOMUM CIRRIGERUM. 285 



come quite separated from the parent sporocyst, and will pro- 

 duce new sporocysts (figs. 4, 5). At other times a sporocyst 

 may apparently divide by fission into two (fig. 7). 



The blastomereSj which appear to have no definite arrange- 

 ment whatever in the embryo, next divide into numerous 

 small cells (fig. 6), and there is produced a thin irregular 

 cuticular covering which cuts off all outside buds, and puts an 

 end to any further multiplication. Thus the sporocyst now 

 consists of an apparently homogeneous mass of very small 

 cells enclosed by a thin cuticle. 



The sporocyst becomes somewhat bean-shaped, and the 

 first sign of differentiation among its minute cells is the for- 

 mation of a transversely disposed tract of cells in the neigh- 

 bourhood of the hilum. This tract, shown in section in figs. 

 8 and 9, becomes cut off by a membrane from the surrounding 

 cells, and some sign of a lumen soon appears. It was at first 

 thought that the structure thus formed was the endoderm, 

 but such is not the case, for it is the beginning of the cirrus- 

 organs, while the gut originates at a later period, and is not 

 produced from these cells. This fundament, however, is often 

 not produced until after the thick cyst-wall, to be mentioned 

 below, is formed. 



The outermost cells of the embryo in contact with the thin 

 irregular cuticle appeared, with the most careful observation, 

 to be identical with the cells further in ; or, in other words, 

 there is at this period no differentiation into either ectoderm 

 or endoderm. 



The sporocyst becomes compressed and convexo-concave, 

 and there is formed a very thick cyst-wall (fig. 10, c. cy.) 

 within the original thin irregular cuticle. In some parts of 

 the embryo this cyst-wall may be formed at some depth 

 within the body, so that some of the outer cells of the 

 sporocyst become cut off completely from the cells within the 

 cyst. This process is not sufficiently regular to be regarded 

 as the casting off of the ectoderm, for on one side of the 

 embryo the cyst-wall may be in contact with the irregular 

 cuticle, while on the other side it may be separated from it 



