86 THE EVOLUTION OF SEX. 



Beneden and others on the fact that, in certain Hydrozoa, "the ova are 

 derived from the endoderm, and the sperms from the ectoderm." Thus 

 Gegenbaur, accepting this, remarks that in such cases "the endoderm is the 

 female and the ectoderm the male germinal layer." Such a generalization, if 

 established, would be plausible enough, seeing that the inner or endoderm 

 layer is the more nutritive or anabolic of the two. A controversy, however, 

 soon arose, the result of which was to overthrow the generalization. In hydra, 

 we have already noticed that both products arise from the ectoderm ; the same 

 was shown by Ciamician to be true of Tubularia mesembryanthemum ; while in 

 the Eudendrium ramosum the ova appeared to arise from the ectoderm, and 

 the male elements from the endoderm, the very reverse of Van Beneden's 

 conclusion. The matter was settled, so far as the general facts are concerned, 

 by Weismann, who established the fact of active migration of the elements 

 from one layer to another. He has since been followed by other investigaters. 

 {a) The sex-elements, both male and female, may appear first in the endoderm, 

 whether they originate there or not, and from this inner layer they migrate 

 to the ectoderm, where they ripen, (b) In rare cases they even ripen in the 

 endoderm. (c) Very commonly the sex-cells originate in the ectoderm and 

 ripen there, or they may pass thence into the endoderm and back again to 

 the ectoderm, (d) In the medusa of Obelia, the ova appear to ripen partly in 

 both layers. These facts, a convenient summary of which will be found in 

 Hatchett Jackson's erudite edition of Rolleston's " Forms of Animal Life," 

 show plainly enough how varied are the origin and history of the sex-cells in 

 these forms. 



The colonial hydroids typically produce well-marked reproductive individuals 

 or sexual zooids, set free as "swimming-bells" or medusoids (in a process to 

 be afterwards described under "Alternation of Generations"). In these the 

 reproductive elements are typically developed. But in varying degrees these 

 medusoids have degenerated, and are frequently not only not liberated, but lose 

 their characteristic features, and become mere reproductive buds. In these 

 buds the sex-cells are normally developed. But it very frequently happens that 

 they arise more or less in the body of the asexual vegetative hydroid. They ripen 

 early, and subsequently migrate to their proper place ; the asexual stage 

 incorporating more and more of the originally separate sexual generation. 

 Weismann has emphasized the value of this early ripening as an advantage to 

 the race, lessening the danger of its extinction ; and this has doubtless to be 

 considered, though it can hardly be regarded as a physiology of the facts. 



VII. Early Separation of Sex-cells. — Having noted the 

 general fact of mesodermic origin, and some of the interesting 

 phenomena observed in ccelenterates, we shall not further pursue the 

 subject except as regards one question, the period at which the repro- 

 ductive cells make their appearance. This is sometimes early, some- 

 times late; and it is not yet decisively known how widely early separa- 

 tion occurs, nor how far the fact is of much significance. The ques- 

 tion will have to be discussed in the volume treating of heredity; 

 only a brief reference is here possible. 



In the case of a well-known fly (Chironomus), Prof. Balbiani, 

 unprejudiced by any theory of heredity, observed the following facts: 

 Before the segmentation of the egg had at all advanced, before what 



