THE DEVELOPMENT OF ASTERINA GIBBOSA. 370 
Figs. 113—117 are intended to illustrate the formation of 
the genital rachis; and they all represent portions of sections 
cut parallel to the disc; those portions, in fact, which are 
transverse sections of one of the five interradial folds of the 
body-wall which in the star-fish project into the body-cavity. 
As we see in Pl. 23, fig. 83, the axial sinus, right hydrocele, 
and the stone-canal, are embedded in one of these folds. It 
follows that the coelomic wall of this particular fold represents 
the larval septum between the anterior ccelom and the pos- 
terior cceloms; and its interradial position in the star-fish 
becomes explained when we remember that the stalk with its 
contained anterior ccelom lies opposite an interradius of the 
water-vascular ring; which interradius is constituted by the 
outgrowth of processes of the two lobes situated at the ends of 
the hydroccele, which is as yet an imperfect ring. These out- 
growths meet, so to speak, above the neck of the stalk. Figs. 
118 and 114 are from the same specimen as fig. 109. We 
see the appearance of the rudiment of the germ cells in a 
section parallel to the adult plane, and notice the remains 
of the cavity of invagination (fig. 114, pr. germ. inv.). 
Fig. 113 shows us that one horn of the right hydroccle has 
become embedded in the ovoid gland, and this is one reason 
why it is extremely difficult to trace the continuity of the 
primitive germ cells by sections taken parallel to the adult 
plane, since the cord of cells is in some spots so narrow, 
and is therefore difficult to distinguish from the epithelium 
lining the right hydrocele. Longitudinal sections, such as 
fig. 104, show it much better. In figs. 115 and 116 (taken 
from a specimen in which R equals *7 millimetre) we see the 
formation of the genital rachis; this takes place by a lateral 
outgrowth from. the primitive patch of invaginated peri- 
toneum, from which we have seen the core of the ovoid 
gland originating as an orally directed outgrowth; the aboral 
sinus which surrounds it (a@d.) is formed at the same time, 
it is a portion of the ccelom shut off by the outgrowth 
of a fold of peritoneum. Fig. 117, taken from a much 
older specimen, shows the genital rachis in its complete form 
