THE UROGENITAL SYSTEM 299 



Coelenterates. Among coelenterates the individual may be hermaph- 

 roditic as in the case of Hydra, or sexually differentiated as in the case of 

 most jelly-fishes. Vegetative methods of multiplication are common in 

 the group. Some have a regular alternation of sexual and asexual methods 

 of reproduction. In the jelly-fishes, eggs and spermatozoa arise in the 

 endoderm and are discharged to the outside through the mouth. For 

 vertebrates a similar endodermal origin of primordial germ-cells has been 

 asserted. 



Flatworms. An advance towards the reproductive organs of chordates 

 is made by the flatworms, in which the gonads take the form of a series 

 of paired gonadic sacs. Sexes in the nemerteans are separate and the 

 gonadic sacs contain either ova or spermatozoa. Germ-cells are prolifer- 

 ated from the epithelial hning of the gonadic sacs, and are discharged to the 

 exterior through paired apertures. It is possible that the metamerism 

 characteristic of vertebrates had its origin in such a series of paired 

 gonadic sacs. (Fig. 268) 



Annelids. The number of metameres in which germ-cells develop 

 is much reduced in anneUds. The region of proliferation of eggs and 

 spermatozoa is also limited to restricted areas of the peritoneal lining 

 of the coelomic cavities. Paired and metamerically arranged coelomo- 

 ducts provided with cihated internal apertures convey the germ-cells to 

 the exterior. 



Protochordates. The gonads of Balanoglossus have a striking resem- 

 blance to those of flatworms. (Fig. 268.) Like the latter, they form a 

 series of paired sacs each of which opens to the exterior by an external 

 aperture. The gonads of Amphioxus also are metamerically arranged in 

 segments 10 to 35. From them the germ-cells escape to the peribranchial 

 cavity and through the posterior atriopore to the exterior. 



Cyclostomes. The gonad of cyclostomes is peculiar in being a median 

 and unpaired organ which extends through nearly the entire length of 

 the body-cavity. In Myxine the right gonad alone persists in the adult. 

 Metamerism is, however, not evident. That vertebrates have metameric 

 gonotomes as well as myotomes has not been demonstrated. It is, 

 however, possible to beheve that the elongated gonads of cyclostomes and 

 of some fishes have been formed by the fusion of primarily separate 

 metameric gonadic sacs of invertebrates. The early gonad of cyclostomes 

 appears to be hermaphroditic, but during ontogenesis changes into either 

 an ovary or a testis. The factors which determine which of the two shall 

 arise are obscure. Sex in cyclostomes, however, appears not to be as 

 definitely predetermined in the chromatin constitution of the fertiHzed 

 egg as it seems to be in higher vertebrates. 



No special reproductive ducts are found in cyclostomes. The eggs 

 collect in the body-cavity and pass to the outside by way of paired abdom- 



