. Elements of Comparative Anatomy. SEXUAL OEGANS OF CCELENTEEATA. 119 Sexual Organs. § 94. Sexual differentiation is not the sole factor in reproduction among the Coelenterata, for various forms of asexual multiplication (cf. supra, §§ 73-77) obtain among them. Sexual products have been observed in most of them, but they are not formed in organs set apart; the function seems rather to be one which is being gradually localised. In the Spongise the endoderm is said to be the place where these products are formed, but in those Porifera which have a mesoderm, the differentiation appears to take pl
. Elements of Comparative Anatomy. SEXUAL OEGANS OF CCELENTEEATA. 119 Sexual Organs. § 94. Sexual differentiation is not the sole factor in reproduction among the Coelenterata, for various forms of asexual multiplication (cf. supra, §§ 73-77) obtain among them. Sexual products have been observed in most of them, but they are not formed in organs set apart; the function seems rather to be one which is being gradually localised. In the Spongise the endoderm is said to be the place where these products are formed, but in those Porifera which have a mesoderm, the differentiation appears to take place in it. The history of the ova is best known; they arise from cells in the mesoderm, but they are perhaps endodermal cells which have passed into it. In addition to what has been directly observed in this group we must bear in mind the characters which obtain in the Hydroid-Polyps (see below). The male elements have been less widely observed. The endoderm has been said to be the place where the seminal cells are formed, but masses of sperm have been observed in the mesoderm of Halisarcaj together with a sexual differentiation of the stocks. § 95. The place where the generative matters are formed—as a rule in the walls of the digestive cavity, or the spaces leading from it—is most exactly known in the Hydroida among the Acale- phfe. The material of the two kinds of generative pro- ducts is, however, provided by different layers of the body : this fact deserves to be exactly described on account of its fundamental import- ance. The first, or indifferent stage, is represented by di- verticula of the wall of the body, which have the form of buds, surrounding a pro- longation of the gastric cavity, and foi'med by the ectoderm and endoderm. A number of the cells of the ectoderm (a) of the growing bud (Fig. 47,^ B) enlarge and become distinguished by their size from the other endodermal cells, which bound the gastric cavity ((/). These enlarged cells are pushed out towards
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