THE UROGENITAL SYSTEM 409 



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. In 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 fining 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. 336) 



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

 is much reduced in annelids. 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 ciliated internal apertures convey the germ-cells 

 to the exterior. In some species, the nephridia join longitudinal paired 

 ducts which in their development and relations resemble the primitive 

 kidney ducts of vertebrates. Their mesodermal origin, however, seems 

 doubtful. Consequently their presence may not be used as evidence to 

 support the annefid theory of vertebrate ancestry. 



Protochordates. The gonads of balanoglossus have a striking resem- 

 blance to those of flatworms. 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. It has been suggested 

 that the relations of the gonads of amphioxus resemble those of the 

 coelomoducts of annelids as well as those of the pronephric tubules of 

 vertebrates. 



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 believe 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 ovarv or a testis. The factors which determine which of the two shall 



