I. URINOGENITAL ORGANS. 



THE urinogenital organs of all Vertebrates arise in the region 

 of the dorsal body-wall, right and left of the middle line. 



The first part to be developed is an unsegmented and paired 

 duct, which arises from the somatic mesoblast, and runs parallel 

 to the long axis of the body. This duct opens anteriorly into 

 the body-cavity by means of one or more ciliated funnel-shaped 

 apertures, which communicate with it by means of convoluted 

 tubes. The latter constitute the pronephros or head-kidney; 

 they are formed as outgrowths of the tube itself, which opens 

 posteriorly into the cloaca, and which is known as the segmental 

 or pronephric duct. 



The pronephros, as far as is known, is always present in types 

 with a larval development {i.e. all Fishes except Elasmobranchs, and 

 Amphibians), but it is usually only transitory. In other types 

 (Elasmobranchii and Amniota) it is practically absent, or at any rate 

 never has any physiological function. 1 The segmental duct, how- 

 ever, persists, and serves to carry off the products of excretion from 

 a second series of glandular segmental tubules, which appear later, 

 and constitute the mesonephros or Wolffian body. This 

 also consists of a series of segmentally- arranged ciliated tubules 

 or nephridia, lying transversely to the longitudinal axis of the 

 body, which arise as buds from the peritoneal epithelium, 



1 Mikalovics has lately shown that the primitive excretory organ in the embryos 

 of the Lizard, Duck, and Fowl consists of two parts, an anterior and a posterior. The 

 former consists of a mmiber of vesicles, the cavities of which in a certain stage of 

 development communicate on one hand, by means of funnels, with the coclome, and 

 on the other with the cavities of the mesoblastic somites. The posterior portion of 

 the organ arises as a series of primitively solid structures in the mesoblastic tissue, 

 which do not communicate either with the ccelome or with the cavities of the somites. 

 The segmental duct arises on the outer and dorsal side of this apparatus, all the 

 constituent parts of which give rise later to hollow tubules, which come to open into 

 the segmental duct, and in each of which a glomerulus is formed. Mikalovics con- 

 siders that the anterior part of the organ corresponds to the pronephros of the 

 Anamnia, and the hinder part to the mesonephros. Sedgwickhas also found traces 

 of a pronephros in chick embryos. We may hope for confirmation of these views in 

 further researches. 



