256 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Coeleuterata. 



Sexual Organs of Hydra.* — According to Prof. Milnes Marshall 

 Hydra is an extremely modified/and not a primitive form, of Hydrozoa. 

 Keferring to the various conditions under which the gonads occur in 

 Podocoryne, Eudendrium, Cordylophora, &c., he gives his reasons for con- 

 sidering the " sporosac," " disguised medusa," and " attached medusa " 

 as due to arrested developments, rather than as stages in the pro- 

 gressive developments of the free-swimming medusa. His reasons 

 for considering Hydra a derived form are summarized by the author 

 as follows : — 1. Hydra is hermaphrodite, which is probably a secondary 

 condition in all animals in which it occurs. 2. Hydra is a fresh- 

 water form ; these are usually derived from marine forms. 3. The 

 ovary is exceptional in that only one ovum ripens, out of many 

 primitive ova. 4. Cordylophora, the other fresh-water genus, is one 

 in which a great deal of shifting of the ova from their original point 

 of formation takes place. 5. The ovary in Hydra differs from that of 

 ordinary Hydrozoa, in consisting only of ectoderm-cells. 



Gastrula and Mesoderm of Ctenophores.t — The fourth portion of 

 Herr E. Metschnikoff's essay on comparative embryology treats of some 

 points in the development of the Ctenophora ; he finds that the 

 gastrula is the result of invagination which appears to be simul- 

 taneously embolic and epibolic. The blastopore in the latter is oral 

 but here the growth of the ectoderm does not proceed from the 

 animal pole, but from a circular rudiment, and we have in consequence 

 in addition to the true blastopore, an upper pseudoblastopore ; this 

 sooner or later closes, and forms the base of the sensory organs. 



The Ctenophora appear to be the only Ccelenterates that have a 

 mesoderm which arises as a special germinal-layer-like rudiment in 

 the course of embryonic development. In Acalephs and Polyps the 

 formation of the mesoderm is postembryonic, and the layer does not 

 arise as a whole. The author discusses these facts in comparison 

 with the coelom theory of the brothers Hertwig, and points out that, 

 on their view, the mesoderm of the Ctenophora is not a mesenchym ; 

 taken in conjunction with what he has observed in Nais, and Eanvier's 

 observation that, in mammals, during an inflammation of the dia- 

 phragm some of the peritoneal cells are capable of taking up foreign 

 bodies, he doubts whether we can speak of " a complete difference 

 between mesenchym and mesoderm." 



Anatomy of the Madreporaria. J — In his first contribution to this 

 subject Mr. G. H. Fowler gives an account of Flabellum patagonichum 

 and Bhodopsammia parallela ; a pair of mesenteries being defined as 

 two mesenteries whose longitudinal muscle-fibres are ranged on their 

 adjacent faces, the new term of entocoele is applied to such part of 

 the ccelenteron as lies between a pair of mesenteries, and the likewise 

 new term of exocoele to those chambers of which one lies between 



* Proc. Manchester Lit, and Phil. Soc, xxiv. (1885) pp. 32-6. 

 t Zeitschr. f. Wiss. Zool., xlii. (1885) pp. 648-56 (IJ pi.). 

 I Quart. Journ. Micr. Sci., xxv. (1885) pp. 557-97 (3 pis.). 



