notochord 



spinal cord 



sensory ganglion 



epoxial muscles 

 horizontal septum 



renal corpuscle 

 lateral vesicle 



nephric duct. 



Mijilerian duct 



peritoneal funnel 



dorsal aorta 



lateral-line sensory organ 

 lateral-line nerve 



sympathetic ganglion 



postcordinol (subcordinal) 

 hypaxial musculature 



gonad with germ cells 

 dorsal mesentery 



Figure 10-51. Cross section of a 33-mm Scyilium canicula showing interrelationships between 

 kidney tubules and genital ridge. (After Goodrich, 1930) 



condition, not the primitive. The more anterior part of the 

 ridge has female potential, while the male section lies more 

 posteriorly. The cortical region, or ventral margin of the 

 fold, is female, while the region of mesenteric attachment, 

 or medullary invasion, is masculine. Central cavities or tu- 

 bular interiors are general; the latter is always the case in 

 the testis. The ray-finned fishes and cyclostomes differ from 

 the other vertebrates in lacking a distinct medullary com- 

 ponent in the gonad. 



The definitive gonads of gnathostomes are associated with 

 ducts of nephric origin except in the actinopterygian fishes. 

 The ducts of this group develop either as a posterior out- 

 growth of a hollow within the gonad ridge or a hollow 

 produced by folding of that ridge. Although the ducts of 

 both sexes involve similar events, these are separate ducts 

 when both gonads are present in a single individual. 



The Miillerian ducts which serve the females of other 

 fishes and tetrapods arise in the chondrichthian from a 

 pronephric funnel and the splitting of the nephric duct 

 from front to back. In higher forms the ostium does not 

 usually appear as an expanded pronephric funnel but prob- 

 ably was derived from one, as is indicated in Neoceratodus. 

 The anterior end of the definitive tube is formed by invagina- 

 tion, at the anterior end of the mesonephric kidney, of a 

 thickened area of epithelium next to the nephric duct. 

 Development proceeds toward the posterior connection with 

 the cloaca. 



That the chondrichthians share Miillerian ducts with the 



higher forms seems quite puzzling in view of the apparent 

 absence of these structures in actinopterygians. This para- 

 doxical situation has caused most zoologists to accept the 

 assumption that the coelomic tubes observed in ray-finned 

 fishes must be, at least in part, Miillerian ducts. Lack of 

 any proof of this assumption leads to the first clear evidence 

 of a gap within the osteichthian fishes. The many similarities 

 in kidney and gonad development between actinopterygian 

 and choanate fish lose significance when it is observed that 

 the sharks are more like one than the other in terms of these 

 reproductive ducts. 



Since it is usually assumed that the primitive condition is 

 one lacking sex ducts (as in the cyclostome, which has bi- 

 lateral gonads), an early radiation of the vertebrates into 

 three lines is suggested: an actinopterygian line which did 

 not utilize the nephric ducts, a chondrichthian line having 

 the usual kinds of ducts as well as Leydig's glands, and a 

 choanate line with typical ducts but without the Leydig's 

 glands. It is necessary, if the evidence of the previous chap- 

 ters is to be given any credence, to view the similarity of 

 shark and choanate as a parallelism. This opinion is sup- 

 ported by their lack of agreement in terms of Leydig's 

 glands as well as by the obvious parallelisms of egg struc- 

 ture, nidamental glands, and viviparity. This parallelism is 

 made possible, in the case of the female, by the availability 

 of pronephric funnels, with their capacity for posterior tu- 

 bular extension, and in the case of the male, by the prox- 

 imity of mesonephric and testicular tubules. 



338 



THE UROGENITAL SYSTEM 



