22 CCELENTEKATA. 



In some cases the ovum while still retaining the constitution last 

 described becomes invented by a very delicate membrane. Such is the 

 constitution of the ripe ovum of Hippopodius gleba amongst the Siphono- 

 phora ' and of the eggs of Geryonia amongst the permanent Medusfe 2 . 

 The ripe eggs of the Ctenophora usually present a similar structure 3 . 

 After being laid they are found to be invested by a delicate membrane 

 separated by a space filled with fluid from the body of the ovum. The 

 latter is composed of two layers, an outer one of finely granular protoplasm 

 and an inner layer consisting of a protoplasmic spongework containing in 

 its meshes irregular spheres. These latter ai*e stated by Agassiz to be of a 

 fatty nature, and it is probable that in most cases where a protoplasmic net- 

 work is present, this alone constitutes the active protoplasm ; and that the 

 substance which fills up its meshes is to be looked on as a form of food-yolk 

 or deutoplasm, though it appears sometimes to have the power of assimila 

 ting the firmer yolk particles. 



The membrane which invests the ovum of many of the Ccelenterata 

 is probably a vitelline membrane. 



The ova of the Hydrozoa take their origin, in most groups at 

 any rate 4 , from the deeper layer of the epiblast (interstitial layer of 

 Kleinenberg). The interstitial cells in the ovarian region form 

 primary germinal cells, and by an excess of nutrition certain of them 

 outstrip their fellows and become young ova. Such ova differ from 

 the full-grown ova already described, mainly in the fact that they 

 have a proportionately smaller amount of protoplasm round the 

 germinal vesicle. They grow to a considerable extent at the expense 

 of germinal cells which do not become converted into ova. 



The ova of man} 7 Ccelenterata undergo changes of a more com- 

 plicated kind before attaining their full development. Of these ova 

 that of Hydra may be taken as the type. The ovary of Hydra 

 (Kleinenberg, No. 9) is constituted of angular flattish germinal cells of 

 which no single one can be at first distinguished from the remainder. 

 As growth proceeds one of the cells occupying a central position be- 

 comes distinguished from the remaining cells by its greater size, and 

 wedge-like shape. It constitutes the single ovum of the ovary. After it 

 has become prominent it grows rapidly in size, and throws out irregular 

 processes. The germinal vesicle, which for a considerable time remains 

 unaltered, also at length begins to grow ; and the sharply defined 

 germinal spot which it contains after reaching a certain size com- 

 pletely vanishes. After the atrophy of the germinal spot, there appears 

 in the middle of the ovum a number of roundish yolk granules. 



1 Metschnikoff. Zeitxchrift f. wiss. Zoologie, Vol. xxiv. 1874. 

 Herman Fol. Jenaische Zeitschrift, Vol. m. 



3 Kowalevsky. "Eutwickluugsgeschichte d. Eippenquallen." Mmoiredel'Acad. 

 Petersbotmt, 1866. And Alex. Apassiz. "Embryology of the Ctenophora." Amer. 

 A<:il. of Scicnt-f mul Arts, Vol. x. No. 111. 



4 The view of van Eeueden, according to which the ova have an endodermal (hypo- 

 hlastic) origin, has been shewn to be at any rate confined to certain groups. The whole 

 i|in'.--.tion <>f the origin of the generative products from the germinal layers in the Coslente- 

 nita is still involved in great obscurity. 



