620 A. G. BOURNE. 



further growth of the gonangiura. After the formation of this 

 circular window in the perisarc, the ectodermic tubercle passes 

 outwards, and the endoderm commences to grow into its lumen 

 and to line its walls. 



The gonangium is now ready for the penetration of the 

 sexual cells. The entrance is partly passive, the result of dis- 

 placement and growth, but partly active, resulting from the 

 actual migration of the ova or spermatospores. 



The time of the descent of the sexual cells is not the same in 

 both sexes. In the males this takes place when the endoderm 

 sends its prolongation into the ectodermic tubercle, the mass 

 of spermatospores then glides slowly upwards towards the 

 opening in the perisarc. In the females the corresponding 

 movement often only takes place somewhat later. 



In certain gonangia the peripheral portion retires from the 

 perisarc, and the cellular mass of a gonangium separates as 

 a blastostyle, properly so called, upon which develope the true 

 gonophores. While the blastostyle is growing in length there 

 forms at the spot where the ovules are, a cul-de-sac, which 

 becomes a gonophore ; the latter separates more or less from 

 the blastostyle, and, finally, remains attached by a short 

 pedicle only. 



In the interior of the gonophore the endoderm growing 

 more rapidly than the ectoderm becomes plicated, and the 

 ovules come to lie in the niches so formed. 



As soon as the eggs arrive at maturity fertilisation takes 

 place, and at the same time the endodermic tube slowly retires 

 from the gonophore at its extremity, and during the maturity 

 of the first gonophore a second is formed. Whence come the 

 ova in the second gonophore ? Are they derived from the 

 coeiiosarc or from the blastostyle ? Weissmann su])ports the 

 latter hypothesis. A third gonophore may be formed, but such 

 rarely happens. 



Weissmann describes similar processes in Plumularia 

 setacea and SertulareUa polyzonias and S. gayi. 



In Gonothyroea Loveni Weissmann stales that the sexual 

 cells do not arise in the coenosarc, but in the gonophores. 



