4IO 



COMPARATIVE ANATOMY 



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

 definitely predetermined in the chromatin constitution of the fertiHzed 

 egg as it is assumed 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- 

 inal pores, which in structure and relations resemble the paired pores 

 of the gonadic sacs of flatworms. The elongated body cavity of verte- 

 brates may be considered as formed by the fusion of the cavities of a 

 similar series of paired coelomic sacs. Indeed, evidence that this assump- 

 tion is correct is furnished by the ontogenesis of the body cavity in 

 amphioxus. In the embryos of this animal each mesodermal somite 



-MESORCHIUM 



A UROGENITAL SYSTEM - RANA B SECTION A-A' - RIGHT GONAD. C SECTION B-B' - LEFT GONAD. 



Fig. 337. — The urogenital system of a young leopard frog showing a stage in the 

 transformation of the primitive gonad into a testis. In the left gonad the metamorphosis 

 is nearly completed. Only the small posterior lobe contains eggs. In the right gonad, 

 which appears like a young unpigmented ovary, some crypts containing spermatozoa 

 have already made their appearance. Most of the lobules, however, are filled with ova 

 as if the gonad were to become an ovary. 



contains a separate cavity (coelom). As the somite extends around the 

 side of the body its ventral wall meets that of the somite opposite below 

 the intestine. By the disappearance of the double membrane thus 

 formed the coelomic cavities of the two somites become continuous. 

 Later, on the ventral side of the body in the trunk region, the series of 

 partitions which separate the cavities of successive somites disappear and 

 a continuous body cavity or coelom extending through many metameres 

 is formed. The extended body cavity of vertebrates is believed to have 

 had a similar phylogenetic origin through the fusion of such serial gonadic 

 sacs as occur in flatworms and nemerteans. This assumption is further 

 justified by the fact that the relations of the abdominal pores in verte- 

 brates resemble those of the gonadic pores of flatworms. Moreover, their 

 function is similar. In petromyzon both the primitive prcnephric ducts 



