26 



CCELENTERATA. 



Special History of the Ovum in different types. 



COELENTERATA. 



(7) Ed. van Beneden. "De la distinction originelle d. testicule et de 1'ovaire.'' 

 Bull. Acad. roy. Bdgique, 3 e serie, Vol. xxxvn. 1874. 



(8) R. and O. Hertwig. Der Organismus d. Medusen. Jena, 1878. 



(9) N. Kleinenberg. Hydra. Leipzig, 1872. 



Amongst the Coelenterata the ova are developed in imper- 

 fectly specialized organs, which are situated in various parts of 

 the body, for the most part in the space between the epiblast 

 and the hypoblast. 



In Hydra the locality where the ova are developed only 

 becomes specialized at the time when an ovum is about to be 

 formed. At one or more points the interstitial cells of the 

 epiblast increase in number and form a protuberance of germinal 

 cells, which may be called the ovary. In this ovary a single 

 ovum is formed by the special growth of one cell. (Kleinenberg, 

 No. 9.) In the free and attached gpnophores of Hydrozoa, the 

 ova appear either around the walls of 

 the stomach, or the radial canals, or 

 around other parts of the gastro-vas- 

 cular canals. 



Their close relations to the gastro- 

 vascular canals are probably determin- 

 ed by the greater nutritive facilities 

 thereby afforded. (Hertwig, No. 8.) 



In the permanent Medusa-forms 

 the ova have similar relations to the 

 gastro- vascular system. Amongst 

 the Actinozoa the ova are usually 

 developed between the epiblast and 

 the hypoblast in the walls of the 

 gastric mesenteries. Amongst the 

 Ctenophora the ova are situated in 

 close relation with the peripheral 

 canals of the gastro-vascular system, ser P rot P lasm - . P->- Central 



d 



p.m 



FIG. 8. RIPE OVUM OF 

 EPIBULIA AURANTIACA. THE 

 GERMINAL VESICLE HAS BE- 

 COME INVISIBLE WITHOUT RE- 

 AGENTS. 



Copied from Metschnikoff, 

 " Entwicklung der Siphonopho- 

 ren." Zdtschrift f. icn'ss. Zoo/., 

 Vol. xxiv. 1874. 



p.d. Peripheral layer of den- 



which run along the bases of the 

 ciliated bands. There are many ex- 



area consisting of a protoplasmic 

 mesh work. 



