ANATOMY AND DEVELOPMENT OF HYDRA. 7 



dissolved. In this way the whole of the included tissue of 

 the testis becomes converted into spermato/oa, which are 

 ultimately discharged through an opening in the summit. 



The formation of the ovarium — which is usually asso- 

 ciated with the testes in one and the same individual — is 

 next described. The interstitial tissue is here also the basis 

 in which the new structure originates. Within a zone which 

 embraces about half the circumference of the body, the cells 

 of this tissue become multiplied and accumulated into groups 

 composed each of a single layer of cells. These cells now in- 

 crease in size, the groups come into union Avith one another, 

 and there is thus formed between the endoderm and the 

 superficial cells of the ectoderm an elongated cellular plate. 

 The cells which belong to the middle of this plate increase 

 still further in size ; in their protoplasm there appear nume- 

 rous, strongly refringent corpuscles, which collect about the 

 nucleus, while the cells themselves become arranged in 

 superposed series, which all converge towards a common 

 central point, thus giving to the organ a striated appearance. 

 The organ thus formed is the ovarium. It must be ad- 

 mitted that in its mode of formation it differs essentially 

 from the gonophores of the marine Hydroida. These are, in 

 all respects, genuine buds, jfossessing a true zooidal inde- 

 pendence, Avhile the spermatozoa-producing and ova-produc- 

 ing bodies of Hydra can scarcely be regarded otherwise than 

 as mere organs. Hydra Avould thus seem to offer an excep- 

 tion to the universality of the generalisation that the prepa- 

 ration of the generative elements among the Hydroida devolves 

 on special zooids. Kleinenberg sees this difficulty, and meets 

 it by the ingenyous supposition that in Hydra the sexual 

 individual represents the gonophore of other hydroids, while 

 those hydra-buds Avhich reach maturity Avithout developing 

 generative elements correspond to their non-sexual or nutritive 

 zooids. 



Another very important point of divergence betAveen the 

 account given by Kleinenberg of the origin of the sexual 

 elements and that maintained by other observers Avill be 

 found in the seat of their origin being here assigned to the 

 ectoderm. 



My OAvn observations on the origin of the sexual elements in 

 the marine hydroids are, on the contrary, quite in favour of 

 their being products of the endoderm. In these hydroids their 

 first foundation shoAVS itself as a thin, homogeneous stratum, 

 lying betAveen the endoderm and ectoderm of the manubrium 

 of the gonophore, and it must be admitted that as yet we have 

 no grounds for referring this to the one membrane more than 



