THE DEVELOPMENT OP ASTBRINA GIBBOSA. 257 



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 PI. XVI, fig. 83, the axial sinus, right hydroccele, 

 and the stone-canal, are embedded in one of these folds. It 

 follows that the ccelomic wall of this particular fold represents 

 the larval septum between the anterior coelom and the pos- 

 terior coeloms; and its interradial position in the star-fish 

 becomes explained when we remember that the stalk with its 

 contained anterior coelom 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 hydrocoele, which is as yet an imperfect ring. These out- 

 growths meet, so to speak, above the neck of the stalk. Figs. 

 113 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 hydrocoele 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, 

 that it is therefore difficult to distinguish it from the epithelium 

 lining the right hydrocoele. 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 {ab.) is formed at the same time, 

 it is a portion of the coelom 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 



