licles deeper in the stroma. When ripe, the follicles rupture 

 into the body cavity. 



The ovary is sparsely spotted with follicles of which the 

 smaller ones are near the free margin. As the eggs become 

 larger they hang down into the body cavity below the level 

 of the free margin (Figure 9-28). The eggs are shelled while 

 still in the follicle; a small micropile, opening through the 

 shell, allows for fertilization. Not all of the large follicles ob- 

 served develop to mature eggs; some develop at the expense 

 of others which atrophy. The mature eggs are shed into the 

 body cavity and leave the body by way of the coelomic 

 pore, which is the same in both sexes. It is an opening be- 

 tween the urinary pore and the anus (Figure 10-27 B). 



Some individuals of Myxine, about 13 per cent, are 

 sterile; they are either without male or female elements in 

 the gonad band or, if these develop, degeneration occurs be- 

 fore mature sex products are formed. 



Myxinids, like the lampreys, are not protandrous her- 

 maphrodites or hermaphroditic. Protandry refers to being 

 male first and later becoming female. Certainly, lampreys 

 are potentially hermaphrodites with development continu- 

 ing into one sex or the other — the exception being sterile in- 

 dividuals where neither sex seems to have the upper hand. 



General observations on cyc/osfomes The cyclostomes re- 

 semble the actinopterygians in that there is little or no 

 nephric contribution to the gonad. They differ in that the 

 bilateral bands, the more primitive condition, of other 



groups are here fused into a single midline mass. Differen- 

 tiation of the early gonad band into anterior female and 

 posterior male parts agrees with the situation of other 

 groups. 



Miillerian ducts are lacking as is any kind of sex duct. This 

 state might be compared with that observed in Amphioxus. 

 However, if one assumes some sort of basic interrelation- 

 ship between the vertebrate kidney and gonad, then com- 

 parisons cannot be made with Amphioxus. This relationship 

 might also suggest that the primitive pathway for the germ 

 cells of both sexes was through the kidney. The kidney 

 pathway, however, appears to be secondary — the cyclo- 

 stomes are thus primitive, while the actinopterygians and 

 the other living gnathosomes represent two pathways to in- 

 creased efficiency in the transport of germ cells. 



General observations on the reproductive system 



As in the case of the excretory system, a comparative 

 analysis of the reproductive system of the various kinds of 

 vertebrates tends to raise questions regarding phylogeny 

 rather than to answer them. Certainly there is a basic re- 

 productive system plan common to all but plastic and open 

 to modification in terms of ducts in the several groups. 



The germ cells appear to arise in much the same way and 

 to migrate into bilateral epithelial folds or germinal ridges 

 in the gnathostomes, or into a single midline or asymmetric 

 ridge in the agnath. The latter appears to be the derived 



anterior clasper protruding from pouch 



anus 

 abdominal pore 



receptacle 



of Miillerian duct 



denticles 

 sperm groove leading into tube 



end view of clasper 



Figure 10-50. Ventral views of pelvic region in male Hydro/ogus, A, and female, C. A, shows both 

 anterior and posterior pelvic claspers of the male; B, end view of posterior pelvic clasper. 



THE REPRODUCTIVE SYSTEM 



337 



