9C0 SUMMARY OF CUnUENT RESEAUCIIES RELATING TO 



tlio geiiital vessel, but in OphUqiliolh ami Ophiothrix there is a single 

 lai"»! or<»an. It is exceedingly probable that the genital orifices are 

 only patent at the moment of sexual maturity. Spermatogenesis has 

 miK'h the sanio history as in Asterids, but while in the latter there is a 

 nucleus cpiite similar to the lymjdiatic nuclei, in the Ophiurids the initial 

 nucleus undergoes a special mode of development which causes it to 

 increase considerably in size. The primordial ova of Ahterids are 

 lymphatic cells, but in Oi)hiurid8 the lymphatic cell of the genital cord 

 undergoes first a special development. 



M. Cnenot disapproves of the union of Asterids and Ophiurids. He 

 believes that Ophiothrix rosuln is the most differentiated type of the 

 latter, as is Astevias glacialls of the former. The Ophiurid presents certain 

 characters of very young Asterids, cliiefly in the digestive tube and the 

 ambulacral and vascular systems. They diller in the calcareous plate 

 \vliich covers the ambulacral groove, and in the greater perfection and 

 specialization of the nervous system. 



The Astcrid, the Ophiurid, and the Sca-Urcliin, taken when quite 

 young, are absolutely similar ; the first have follosved most directly the 

 common phylum wlieuce all are derived. The Urchins have become 

 more specialized than the Ophiurids, from the stock of which the Euryalids 

 broke oif to become more specialized than the rest. The want of 

 personal study of Holothurians and Criuoids, and the insufficiency of our 

 knowledge coucerning these, have caused the author to omit them from 

 his comparisons. 



Development of Comatula.* — Dr. J. Barrois has investigated the 

 development of Comatula mediterranea. The process lasts a week ; the 

 first day includes segmentation and blastosphere formation ; the second 

 witnesses tlie establishment of gastrula and blastopore ; the third is 

 marked by the formation of cnterocuile and intestine ; the fourth finishes 

 the development of the visceral mass ; the fifth takes efiect in the dis- 

 placement of this mass ; the sixth reveals the formation of the skeleton ; 

 and the seventh is the day of hatching. 



The segmentation results, at the 32 stage, in a mass with eight sides, 

 including four blastomeres each. It exhibits eight large blastomeres 

 (endodermic) at the inferior pole, and a cavity opening to the exterior, by 

 a slit between the blastomeres at each pole. The resulting hlastula loses 

 these two distinctive features of the dividing mass. The gastrula is a 

 typically iuvaginated archigastrula. With the closure of the blastopore at 

 the end of the second day, the cells of the endoderm become disposed in 

 several rows, of which the most exterior form the mesenchyme. The 

 endodermic sac becomes completely detached from the ectoderm, and is 

 divided by a constriction into two superposed vesicles. I'he cells of 

 the mesenchyme continue to accumulate on the low'er portion of the 

 embryo, which becomes distinguishable into two parts : — the upper part 

 (future cal3'x) contains the endodermic sac, the lower i>art (future calyx) 

 contains the mesenchyme. The enteroccele is formed at the beginning of 

 the third day. The superior vesicle, just alluded to, elongates ; the 

 inferior vesicle gives off two horns, which surround it — a small transitory 

 one on the ventral side and a larger dorsal one which will form tlie 

 intestine. The endoderm is now more delicate than the ectoderm. The 

 latter exhibits on its ventral surface a thickening, which is the first 



* Rec. Zool. Suisse, iv. (1888) pp. 543-C51, G pis. 



