48 



EMBRYOLOGY 



Hj'dra as a polyp that has reached sexual maturity, and therefore as a 

 very primitive form of hydroid. 



In general the sexual organs, both in the medusas and in the sessile 

 gonophores, lie in the ectodermal wall of the manubrium (Fig. 18) or 

 (Leptomedusae. Haeckel) on the inner wall of the bell in the course of 

 the radial canals. 



The investigations of Weismann have yielded new results concerning 

 the early stages in the formation of the sexual products. In the 

 original condition the sexual cells were developed and reached maturity 

 in the ectoderm of the manubrium of the medusa. In other cases (in 

 forms with sessile gonophores) the development of the sexual cells took 

 place even before the gonophore itself was fully formed ; there ensued 

 therefore a secondary displacement (phyletic) of the germarium, first 



Fig. 18. — Diagrammatic section through two sexual hydroid individuals. A, 



young medusa, still attached ; B, sessile gonophore ; ov, gonads Covarium) ; in, 

 manubrium ; r, radial vessel ; f, tentacle ; v, velum ; g, vascular lamella. 



into the ectoderm of the bud, then into the entoderm of the same, and 

 finally into the entoderm of the stem and the branches before the de- 

 velopment of the bud. From this location the sexual cells are compelled 

 to migrate (ontogenetically) to the seat of their maturation. It is to be 

 seen that this displacement of the germarium was established in the 

 interest of the greatest possible acceleration of sexual maturity. In this 

 case the displacement (phyletic) of the germarium is in centripetal 

 direction. In the Leptomedusre (Haeckel), on the contrary, the seat of 

 maturation is displaced in centrifugal direction, for Haetlaub has been 

 able to show in Obelia that the sexual cells arise in the manubrium, and 

 reach the radial canals only secondarily. 



Reproduction by budding occurs not only in hydroid polyps, but also 



