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 the 



Fig. 47. Two female generative buds of Hy- 

 dractinia ecliinata. a Ectoderm, h Endo- 

 derm. g Gastric cavity, o Ovarian germs. 

 In A the ectoderm has begnn to bo pushed into 

 the endoderm. In B the invaginated portion 

 has been constricted off from the rest of the 

 ectoderm (after Ed. van Beneden). 



