It has been already stated in the introduction that the male and female generative products are homodynamous, but the consideration of the development of the products in the two sexes shews that a single spermatozoon is not equivalent to an ovum, but rather that the whole of the spermatozoa derived from a spermospore are together equivalent to one ovum.
[12] In all the Metazoa the generative organs are placed between the primitive germinal layers; and the peculiarity of their position in the Cœlenterata depends on the absence of a body cavity and of a distinct mesoblast.
[13] For details on the yolk nucleus vide Balbiani, Leçons s. l. Génération d. Vertébrés. Paris, 1879. In this work the author maintains very peculiar views on the nature and function of the yolk nucleus, which do not appear to me well founded.
[14] In the germinal vesicles of very young ova the reticulum is often absent.
[15] A very complete and critical account of the literature is contained in this paper.
[16] Metschnikoff. Zeitschrift f. wiss. Zoologie, Vol. XXIV. 1874.
[17] Herman Fol. Jenaische Zeitschrift, Vol. VII.
[18] Kowalevsky. “Entwicklungsgeschichte d. Rippenquallen.” Mémoire de l’Acad. Pétersbourg, 1866. And Alex. Agassiz. “Embryology of the Ctenophoræ.” Amer. Acad. of Science and Arts, Vol. X. No. 111.
[19] 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 Cœlenterata is still involved in great obscurity.
[20] The above description of the ova of the Tubularidæ is founded on sections of the gonophores of Tubularia mesembryanthemum. Dr Kleinenberg informs me however that the absence of a distinct boundary between the germinal cells and the ovum is not usual.