THE OVUM. 27 



amples amongst the Ccelenterata of ova which retain in their 

 mature state the very simple constitution which has been de- 

 scribed as characteristic of all young ova ; and which are, when 

 laid, absolutely without any trace of a vitelline membrane or 

 chorion. In many other cases both amongst the Medusae, the 

 Siphonophora, and the Ctenophora, the ripe egg exhibits a dis- 

 tinction 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.) 



In some cases the ovum while still retaining the constitution last 

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

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

 phora 1 and of the eggs of Geryonia amongst the permanent Medusae 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 are 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 



1 Metschnikoff. Zeitschrift /. wiss. Zoologie, Vol. xxiv. 1874. 



2 Herman Fol. Jenaische Zeitschrift, Vol. vil. 



3 Kowalevsky. ' ' Entwicklungsgeschichte d. Rippenquallen." Memoire de 

 FAcad. Petersburg, 1866. And Alex. Agassiz. "Embryology of the Cteno- 

 phorEe." Amer. Acad. of Science and Arts, Vol. x. No. in. 



4 The view of van Beneden, according to which the ova have an endodermal 

 (hypoblastic) origin, has been shewn to be at any rate confined to certain groups. 

 The whole question of the origin of the generative products from the germinal layers 

 in the Ccelenterata is still involved in great obscurity. 



