XVI ECHINODEEMATA 481 



projects iato this fold. The involution, at first hollow, soon becomes 

 solid and is termed the primary germinal involution (p.g.inv, Fig. 

 371, A). The solid bud thus produced proliferates downwards into the 

 axial organ and forms the peculiar cells characteristic of this structure, 

 but it also proliferates laterally, to the right and to the left, and 

 forms a cord of primitive germ cells known as the genital rachis 

 (^g.r, Fig. 371, B), which grows as a freely projecting rod right round 

 the disc of the star-fish until the two ends meet and a complete 

 circle is formed. The rod is, of course, covered with a thin layer of 

 peritoneum which is reflected over it ; it is supported in a sling of 

 peritoneum which grows out parallel with the rod and underneath 

 it, as a flap, which fuses with the body wall on both sides and 

 encloses a space called the aboral sinus (ab.s, Fig. 371, B). 



At each side of the base of each arm a branch is given off from 

 the rachis enclosed in a branch of the aboral sinus. This branch 

 extends downwards and then along the arm, where it enlarges and 

 forms a bunch of diverging branches which constitute the genital 

 organ. Eound each branch, of course, is a branch of the aboral 

 sinus, but this branch becomes cut off from the main part of the 

 sinus by the ingrowth of a septum (gon, Fig. 371, C). The genital 

 duct is formed as a solid outgrowth from the base of the genital 

 organ, where it joins the rachis. This outgrowth fuses with the 

 ectoderm and then becomes hollowed out. The process by which 

 a genital gland discharges itself bears a considerable resemblance to 

 the way in which an. abscess finds its way to the surface. 



Both Solaster and Gribrella agree, as far as the general form of 

 their larvae is concerned, with Asterina, though differing in details of 

 shape and size of the prae-oral lobe, and they have a quite similar 

 metamorphosis. But they differ in one most important respect, 

 they never develop a larval mouth, and the larval gut remains in 

 a most rudimentary condition, being merely a collar of more 

 columnar cells round the middle portion of the archenteron. The 

 hinder portion of the archenteron gives rise to the left posterior 

 eoelom, whilst the front portion gives rise, not only to the single 

 anterior eoelom, but also to the right posterior eoelom. From the 

 anterior eoelom are also given off the hydrocoele and the madreporic 

 vesicle. 



The credit of elucidating this extraordinary form of development 

 belongs to Masterman (1902), whose observations on Gribrella have 

 been confirmed in almost every point by those of Gemmill on 

 Solaster (1912). Masterman (1902), viewing this development as 

 primitive, regards the right posterior eoelom as the antimere of the 

 left hydrocoele, and the left posterior eoelom as a median structure 

 containing right and left elements. He terms the madreporic vesicle 

 the central eoelom. According to Gemmill, in Solaster the madre- 

 poric vesicle arises to the right of the middle Une, as in Asterina. 



There are obvious objections to regarding the shortened develop- 

 ment of an Asteroid, with a yolky egg, as capable of throwing light 



VOL. I 2 I 



