THE DEVELOPMENT OP ASTERINA GIBBOSA. 259 



sinus [gen. r., woodcut 1) ; one of these branches runs in an oral 

 direction down each side of the interradial septum. This 

 septum is an ingrowth of the body-wall, which has by this time 

 become marked, though its first beginnings date back to the 

 end of the metamorphosis (PI. XVI, fig. 84). 



A section of one of these branches in an older specimen is 

 given in PI. XIX, fig. 119). These genital branches are formed 

 as the rachis reaches each interradial septum before it has 

 formed a circle ; in one specimen I have observed a rachis 

 reaching only to the next interradius, and there giving off one 

 genital branch. Figs. 120 and 121 (taken from the same 

 specimen as fig. 119) show the first rudiments of the genital 

 organs. The branch of the rachis ends in a swelling accom- 

 panied by a dilatation of the aboral sinus, and we see the begin- 

 ning of a septum tending to shut off the main aboral sinus from 

 this dilatation. This septum was first described by Cuenot (3), 

 and in it the genital duct is formed. This is shown in fig. 123, 

 taken from the oldest specimen I examined, in which R equals 

 3'7 millimetres. We see that the genital duct is formed by 

 a core of primitive germ cells burrowing its way through the 

 body-wall. Fig. 122, from the same specimen, shows the con- 

 tinuity of the rachis and the ovary. We notice also the forma- 

 tion of follicles from the indifferent germ cells. 



We are now in a position to compare the arrangement of 

 the ovoid gland and genital rachis and their accompanying 

 spaces in Amphiura squamata with that found in Asterina 

 gibbosa. In the former I described the genital rachis issuing 

 from the oral end of the gland and accompanied by three 

 spaces, which I named sinus a, sinus h, and sinus c (PI. XVIII, 

 fig. 112). This figure is a diagram of a section parallel to the 

 long axis of the stone-canal. Fig. Ill is a diagram of a 

 similar section of Asterina, but it is not quite accurate, since it 

 shows both the ovoid gland and the stone-canal, and these two 

 structures do not lie in the same radial plane in Asterina. 

 In order to avoid obscuring the opening of the stone-canal into 

 the axial sinus, it is necessary to indicate part of the ovoid 

 gland by dotted lines. 



