8 • DUBLIN 



error into which it is easy to fall in considering the sex condi- 

 tions oi P. americana, viz., to suppose, in the light of the Men- 

 delian principles expressed in Castle's paper on the Heredity of 

 Sex, that there is an alternation of dominance and recession of 

 one sex or the other. This interpretation is made impossible 

 from the conditions described in the allied forms and one is 

 forced to the conclusion already adopted above, that there is a 

 true mosaic of sex in which the elements are widely scattered 

 in time and place. The colony as a whole, is now an hermaph- 

 roditic individual. 



III. Spermatogenesis. 



The testis of Pediccllina ainericana is a paired bilaterally 

 symmetrical organ situated in the space between the liver cells 

 and the atrium. At the center, between the two component 

 halves, lies the vas deferens, through which the ripe spermatozoa 

 make their way directly into the atrium and thence to the out- 

 side. The two halves are pear-shaped with the broader ends 

 closely apposed to the body wall of the polyp, from which they 

 are separated by a thin layer of epithelial cells surrounding the 

 entire structure. It is interesting, in the study of the living 

 individuals, to observe the mechanism by which the extrusion 

 of the spermatozoa is accomplished. Upon slight provocation, 

 such as touching the polyp with a needle point, the tentacles 

 are immediately drawn in, and simultaneously the calyx is much 

 contracted. This is followed by an extrusion of the ripe sper- 

 matozoa in dense clouds, through the atrium. During this proc- 

 ess of extrusion, also, the polyp is subjected to a change of 

 position by the rapid movements of the stalk. 



In a longitudinal or transverse section of such a polyp, the 

 internal relations of the testis can be very readily made out. 

 There is nothing corresponding to a subdivision of this organ 

 into compartments, as in so many forms ; nor is there any 

 arrangement of the germ cells with respect to age, the cells 

 lying scattered irregularly throughout. The great number of 

 developing cells is very striking. In a fairly large individual 

 the testes filled with densely crowded cells take up by far the 

 greatest portion of the calyx, and compress on all sides the 

 other organs of the body. 



