118 On the Structure and Development of Lepidosteus. [Dec. 8, 



is required before such homology can be considered as definitely 

 established. 



Under the third heading the skeletal elements supporting the fin- 

 rays of the ventral lobe of the caudal fin of various types of fishes are 

 compared and the following conclusions are arrived at. 



(1.) The ventral lobe of the tail-fin of Pisces differs from the other 

 unpaired fins in the fact that its fin-rays are directly supported by 

 spinous processes of certain of the hsenial arches, instead of by in- 

 dently developed interspinous bones. 



(2.) The presence or absence in the tail-fin of fin-rays, supported by 

 hpamal arches, may be used in deciding whether apparently diphycercal 

 tail-fins are aborted or primitive. 



Urogenital Organs. 



With reference to the character of the adult urogenital organs, the 

 authors show that for the female the descriptions of Muller and Hyrtl 

 are substantially accurate, but that Hyrtl's description of the genera- 

 tive ducts of the male is wholly incorrect. 



They find that in the male the semen is transported from the testes 

 by means of a series (40 — 50) of vasa efferentia, supported by the 

 mesorchium. In the neighbourhood of the kidney these vasa unite 

 into a longitudinal canal, from which transverse trunks are given off, 

 which become continuous with the uriniferous tubuli. The semen is 

 thus transported through the kidney into the kidney-duct (segmental 

 duct), and so to the exterior. ~No trace of a duct homologous with the 

 oviduct of the female was found in the male. 



With reference to the development of the excretory system, the 

 authors have established the following points : — 



(1.) That the segmental (archinephric) duct is developed as in 

 Teleostei. 



(2.) That a pronephros, resembling in the main that of Teleostei, 

 is developed from the anterior end of the segmental duct. But they 

 find that the pronephric chambers, each containing a glomerulus, 

 into which the coiled pronephric tubes open, are not, as in Teleostei, 

 completely shut off from the body cavity, but remain in communica- 

 tion with it by two richly ciliated canals, one on each side of the 

 body. 



(3.) The pronephros eventually undergoes atrophy. 

 (4.) Some of the mesonephric tubes have peritoneal funnels in the 

 larva. 



(5.) The ovarian sac continuous with the oviduct, is established 

 by a fold of the peritoneal membrane, near the attachment of the 

 mesovarium, uniting with the free edge of the ovarian ridge to form a 

 canal, the inner wall of which is constituted by the ovarian ridge 

 itself. 



