452 ECHINODERMATA ASTEROIDEA chap. 



vessel, and is termed the " genital rachis " (Fig. 192, g.r). The 

 germ-cells are distinguished by tlieir large nuclei and their 

 granular protoplasm. The genital organs are only local swellings 

 of the genital rachis, and from the shape of some of the germ- 

 cells it is regarded as highly probable that the primitive germ- 

 cells wander along the rachis and accumulate in the genital 

 organs. The genital rachis itself is an outgrowth from the 

 genital stolon, and this latter originates as a pocket-like ingrowth 

 of the coelom into the wall separating it from the axial sinus ; 

 when fully formed it projects into and is apparently contained 

 in this latter space. 



Not all the cells forming the genital stolon become sexual 

 cells. Many degenerate and become pigment-cells, a circumstance 

 to which the organ owes its brown colour. In very many species 

 of Starfish many of the cells of the genital rachis undergo a 

 similar degeneration, and hence is produced the apparent aboral 

 blood-vessel. Further, the rachis is embedded in connective 

 tissue which has undergone what we may call the " lymphatic " 

 modification, and this for want of a better name we call the 

 " aboral " blood-ring. 



The size of the genital organs varies with the season of the 

 year ; they are feather-shaped, and attached to the genital rachis 

 by their bases, but project freely into the coelom of the arm. 

 From their great variation in size and also from the shape of 

 some of the cells in the genital rachis, Hamann concludes that as 

 each period of maturity approaches fresh germ-cells are formed 

 in the rachis and wander into the genital organ and grow there 

 in size. It is probable that the aboral end of the genital stolon 

 is the seat of the formation of new germ-cells. 



In the Starfish, therefore, as in other animals with a well- 

 defined coelom, the genital cells ultimately originate from the 

 coelomic wall. 



The genital ducts are formed by tlie burrowing outwards of 

 the germ-cells. When it is remembered that tlie fundamental 

 substance of the body-wall is semi-fluid jelly, this process will be 

 better understood. 



When the ova and spermatozoa are ripe, they are simply shed 

 out into the sea and fertilisation occurs there. The development 

 is described in Chapter XXI. The free-swimming larval period 

 lasts about six weeks. 



