372 E. W. MACBRIDE. 
one at each side of each arm. Local thickenings of these 
branches of the rachis constitute the genital organs. The 
surrounding spaces, the genital sinus (ad 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 StageG. We see the rudiment of the ovoid gland 
(ovg.) 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 
celom (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 perihemal 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. 367). 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 hydroccele 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 perihemal 
canal (figs. 105 and 106). 
Pl. 25, fig. 110, which represents a similar section to figs. 
99—106, shows practically the adult condition of the ovoid 
