REPRODUCTION. 



149 



Fig. 00. 



This organ forms the axile diverticulum in the young adclocodonic gonophorc, and the 

 manubrium of the sexual medusa, while it is represented by the entire sexual zooid which buds 

 from the radiating canals in the blastocheme or non-sexixal medusa. 



It is not at first easy to say whether the generative elements have their proper origin in the 

 ectoderm or endoderm of this body, as in most cases they can be merely seen filling the 

 space between these two membranes, which become more and more separated from one another 

 iis the included mass of ova or spermatozoa increases in volume. 



From some favorable observations, however, which I have succeeded in making on certain 

 species of hydroids, I have convinced myself that the true origin of the ova and spermatozoa is 

 to be found in the endoderm, while the ectoderm serves merely as a confining and protecting sac 

 until such time as the generative elements acquire sufficient maturity to allow of their liberation, 

 which always takes place by simple rupture or absorption of the ectodermal sac. 



Thus, in the gonophores of the male colonies of Sertularia polyzoiiias the spermatogenous 

 tissue may be seen filling the entire space between the long cylindrical axile spadix and the 

 surrounding walls of tiie gonophore. In most specimens it may be easily seen that the sperma- 

 togenous mass is far from being of uniform maturity throughout ; for while towards the axis of 

 the gonophore it is still very inuuature, the mother-cells being here distinctly visible with the 

 ultimate spermatic cells within them, we find that towards the periphery it consists of free active 

 spermatozoa. The youngest portion of the mass is thus that which is still in contact with the 

 spadix or endodermal portion of the gonophore, while the oldest portion is situated externally, 

 being in contact with the confining ectoderm — a condition 

 which would be scarcely possible if the ectoderm, rather 

 than the endoderm, gave origin to the spermatic cells. 



A state of things exactly parallel to this may be seen 

 in the female gonophores of Cori/ne pmiUa, in which, 

 moreover, the actual formation of the ova may be 

 satisfactorily traced. At an early period in the deve- 

 lopment of these gonophores, the large thick spadix 

 may be seen to be surrounded by a granular plasma, 

 throughout which numerous minute nucleated cells are 

 scattered (woodcut, fig. GO). These cells I regard as 

 the germinal vesicles and spots of the future ova, round 

 which no distinctly differentiated vitellus can as yet be de- 

 tected. In a more mature stage of the gonophore, while 

 the same peculiar tissue continues to invest the spadix, 

 the peripheral portion of this tissue may be seen to be 

 thrown off in the form of undoubted ova, consisting each of 

 a germinal vesicle and spot precisely similar to those ob- 

 served in the more central portion of the mass, but now 

 with a portion of the common plasma differentiated round 

 each germinal spot in the form of a a very definite vitellus. 

 AVhen the gonophore has attained complete maturity, the 



whole of the jjlasmatic mass, with its immersed nucleated cells, has become metamorphosed 

 into fullv formed ova. 



Young Spoi-osac of Coryn 



, show 



certidn early stages in the foruiatiou of the 



a, Outer wall f perigonium) of the sporosac ; 

 b, cavity of spadix ; c, plasma, investing the 

 spadix, and having imbedded iu it the germinal 

 vesicles of the future ova; within the germinal 

 vesicles are seen the germinal spot and the 

 puttctum gprmanitivnm ; d, fully formed ova, 

 in each of which a portion of the common 

 plasma has become tlidercntiated as a vitellus 

 round the germinal vesicle. 



