634 IffiPOKT OF COMMISSTONEU OF FTSH ATTD^FTSTTEUIFS. [80] 



ably occurs at the posterior dorsal part, as in Cyhium. The allantoic 

 or urinary vesicle opens either into a cloaca or into the extreme hinder 

 and possibly cloacal portion of the anal end of the intestine, as in Alosa, 

 Sabno, and Hippocamjms; a cloaca is, however, probably fully developed 

 at a later stage, into which the generative ducts, bladder, and intestine 

 open. Upon the development of the cloaca I have made very insuffi- 

 cient observations, and whether it is developed from the anal end of the 

 larval intestine and lower end of the allantoic sack I am not able to 

 state. While in some forms there is an emargination of the ventral 

 fin-fold where the intestine ends, there is as yet no common external 

 depression in which the alimentary, genital, and urinary canals termi- 

 nate; this structure must therefore be relatively late in developing. 



The segmental ducts are simple, straight cylindrical canals through- 

 out, exempt at the anterior extremity, the walls of which are composed 

 of a single layer of cells. They constitute the simplest expression of 

 the renal excretory system of the vertebrates, and are not provided 

 with any Malpighian bodies or other accessory excretory organs at the 

 stage of development now under discussion, as there is as yet no circu- 

 latory system to supply blood to any glomeruli, even if these were de- 

 veloped. The only place where it may be supi)osed that anything like 

 a glandular character has been acquired by the organ, is at its anterior 

 end, where it is bent upon itself in the peculiar manner already described. 

 The diversity of manner, however, in which we tind the accessory- organs 

 developed in the adults of different genera, as well as the relatively late 

 or early development of these structures in diliereut forms, is no less 

 interesting than the fact that the glandular portion is at first formed in 

 different regions in the embryos of dissimilar genera. For instance, 

 Salmo is in marked contrast with Oarnhnsia in that the mesouophric 

 glomeruli are developed from a little behind the pectoral fins almost to 

 the allantoic vesicle or urinary bladder even before hatching, while in 

 the latter genus the mesonephric portion is quite anterior, and is crowded 

 forward against the auditory vesicle. In still other instances the pro- 

 nephros does not extend nearly as far forward as in either of these 

 ca«es; of this wo have an illustration in the embryos of Alosa, where the 

 pronephros ends far short of the head, but its anterior ternjination is 

 similar in form to that observed in other families. Balfour has noticed 

 some of the conditions of the organ in the adults, and he observes (Comp. 

 Embryo!., II, 579) : " In some cases the cephalic portion of the kidneys is 

 absent in the adult, which probably implies the atrophy of the prone- 

 phros ; in other instances the cephalic portion of the kidneys is the only 

 part developed." This has its significance, and it is imi)ortant tijat the 

 peculiarities of different genera in respect to the mode of origin of the 

 renal organs be investigated. In A^oa'« there is no evidence of glome- 

 ruli on tiie inner side of the segmental ducts until long after hatching; 

 such also appears to be the case with Gaclus. In Oambusianiid Salmo, on 

 the other hand, the segmental tubes are already developed, in the first case 



