SEXUAL ORGANS AND TISSUES. 59 



colonies for itself usually in quite a different way, namely, by 

 the process of sexual reproduction. Among the cells of the 

 middle stratum of the sponge body certain well-nourished 

 passive cells appear. These are the ova, at first very like, but 

 eventually well marked from the other constituent units of the 

 layer. Besides these there are other cells, either in the same 

 sponge or in another, which exhibit very different characters. 

 Instead of growing large and rich in reserve material like the 

 egg-cells or ova, they divide repeatedly into clusters of infini- 

 tesimal cells, and form in so doing the male elements or 

 spermatozoa. The male and female cells meet one another, 

 they form a fertilised ovum ; the result is continued division of 

 the latter till a new sponge is built up. Here then there are 

 special reproductive cells, quite distinct from those of the 

 "body"; and here, furthermore, these reproductive cells are 

 markedly contrasted as male and female elements. As yet, 

 however, there are no sexual organs. 



Passing to the next class, the stinging animals or coelenter- 

 ates, we find in one of the simplest and most familiar of these, 

 the common fresh-water hydra, a good illustration of primitive 

 sexual organs. As in sponges, a cut-off fragment of the body, 

 if sufficient samples of the different component cells are in- 

 cluded, is able to reconstitute the whole. But no one body-cell 

 has of course any such power ; this is possible for the fertilised 

 ovum alone. Now this ovum occurs, not anywhere within a 

 given layer as in sponges, but always near one spot on the body. 

 Towards the base of the tube a protuberance of outer layer 

 cells is developed. I'his forms a rudimentary ovary, or female 

 organ. It has this peculiarity, not however unique, that while 

 the organ consists of not a few cells, only one of these becomes 

 an ovum. A. similar protrusion, or more than one, often at the 

 same time and on the same animal, may be recognised further 

 up the tube, nearer the tentacles of the hydra. Of somewhat 

 smaller size, such a superior protuberance consists of numerous 

 small cells, most of which, multiplying by division, form male 

 elements or spermatozoa. We have here the simplest possible 

 male organ or testis. 



More elaborate organs occur in the other coelenterates, 

 complicated however by two interesting facts, which will be 

 afterwards discussed. {a) Many of the coelenterates are well 

 known to form elaborate colonies, — zoophytes, Portuguese men- 

 of-wnr, and the like. In these, division of labour frequently 



