THE OVUM. 



21 



Special History of the Ovum in different types. 



CCELENTERATA. 



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

 Bull. Acad. roy. Belqique, 3" serie, Vol. xxxvn. 1874. 



(8) B. and 0. Hertwig. Der Organismus d. Medusen. Jena, 1878. 



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



Amongst the Ccelenterata the ova are developed in imperfectly 

 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 

 gonophores of Hydrozoa, the ova appear either around the walls of 

 the stomach, or the radial canals, or around other parts of the 

 gastro-vascular canals. 



Their close relations to the gastro-vascular canals are probably 

 determined 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 peri- 

 pheral canals of the gastro-vascular 

 system, which run along the bases of the 

 ciliated bands. There are many ex- 

 amples amongst the Ccelenterata of ova 

 which retain in their mature state the 

 very simple constitution which has been 

 described as characteristic of all young 

 ova; and which are, when laid, abso- 

 lutely without any trace of a vitelline 

 membrane or chorion. In many other 

 cases both amongst the Medusa?, the Si- 

 phon ophora, and the Ctenophora, the ripe 

 egg exhibits a distinction into two parts. 

 The outer part is composed of a dense 

 protoplasm, while the interior is composed of a network or more 

 properly a spongework of protoplasm enclosing in its meshes a more 

 fluid substance. (Fig. 8.) 



FIG. 8. RIPE OVUM OF EPI- 

 BULIA AURANTIACA. THE GER- 

 MINAL VESICLE HAS BECOME 



INVISIBLE WITHOUT REAGENTS. 



Copied from Metschnikoff, 

 "Eutwicklung der Siphono- 

 phoren." Zeitschnft f. wiss. 

 Zool., Vol. xxiv. 1874. 



p.d. Peripheral layer of den- 

 ser protoplasm, p.m. Central 

 area consisting of a protoplasmic 

 meshwork. 



