254 E. W. MAOBRIDE. 



one iat each side of each arm. Local thickenings of these 

 branches of the rachis constitute the genital organs. The 

 surrounding spaces, the genital sinus [ab gon, figs. 122 and 123), 

 is shut off from the aboral sinus by the outgrowth of a septum. 

 Fig. 99 is the marginal portion of a section vertical to the disc 

 of a larva of Stage G. We see the rudiment of the ovoid gland 

 (fiVff.) as a fold projecting into the axial sinus. Further up we 

 notice a thickened patch of peritoneum, which is invaginated 

 into the septum separating the axial sinus from the left posterior 

 coelom {pr. germ. inv.). This is the rudiment from which, on 

 the one hand, the genital rachis and, on the other, the core of 

 the ovoid gland are derived. Figs. 100 — 103, similar sections 

 to fig. 99, from a just metamorphosed star-fish, illustrate this. 

 We see that from this rudiment a cord of primitive germ 

 cells has grown out and filled the fold which is the 

 rudiment of the ovoid gland. The last two sections 

 cut a more oral portion of the fold, since they are slightly 

 oblique ; we see (figs. 102 and 103) that this core has not as yet 

 penetrated to the oral end of the fold, and, further, that the 

 fold is attached to the oral side of the inner perihsemal ring, 

 or, in other words, that it traverses the lower end of the axial 

 sinus, and is attached to its lower side. The original invagina- 

 tion to form the germ cells is situated at the very tip of the right 

 dorsal horn of the left coelom, where it meets the right ventral 

 horn, but at this level the two horns do not open into each 

 other (see p. 249). Figs. 104 — 106, again representing sections 

 vertical to the margin of the disc, are taken from a young 

 star-fish, in which R equals "4 millimetre. Fig. 104 shows the 

 cord of cells which arises from the peritoneal invagination and 

 penetrates the dorsal organ, and the relation of this cord to 

 the right hydrocoele and the axial sinus. We see that now this 

 core of cells reaches to the oral end of the ovoid gland, and 

 penetrates also a prolongation of the same, which is prolonged 

 as a fold, hanging from the aboral wall of the inner perihaemal 

 canal (figs. 105 and 106). 



PI. XVIII, fig. 110, which represents a similar section to figs. 

 99 — 106, shows practically the adult condition of the ovoid 



