SEXUAL ORGANS AND TISSUES. 



53 



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 coelenterates, 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 included, is able to reconstitute the 

 whole. But no one body-cell has of course any such power; this is 

 possible for the fertilized ovum alone. Now this ovum occurs, not 

 anywhere within a given layer as in sponges, but always near one spot 

 on the body. Toward the base of the tube a protuberance of outer- 

 layer cells is developed. This 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 recognized 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, multiply- 

 ing 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-war, and the like. In these, 

 division of labor frequently goes further than the setting apart of 

 special organs. Entire individuals become reproductive ' ' persons ' ' 

 (as they are technically called), in contrast to the nutritive persons of 

 the colony. {U) In some of those reproductive individuals, a curious 

 phenomenon, known as migration of cells, has been observed by 

 Weismann and others. The reproductive cells, arising in various 

 parts of the body, have been shown to migrate in some cases to 

 another part, where they find final lodgment in more or less definite 

 organs. This occurrence is intimately associated with ' ' alternations of 

 generation," and will be afterwards discussed under that heading. 



It is in no wise the purpose of the present work to describe the 

 details respecting the ovaries and testes, as they occur in the various 

 classes of animals. It is enough for our purpose to have emphasized 

 the fact of their gradual differentiation, and to note that they are al- 

 most always developed in association with the middle layer of the 

 body, and usually occupy a posterior position on the wall of the body- 

 cavity. The details will be found in any standard work on com- 

 parative anatomy, — very conveniently, for example, in Prof. Jeffrey 

 Bell's " Comparative Anatomy and Physiology," London, 1885. 



