I. URINOGENITAL ORGANS. 



a. GENERAL PART. 



The first traces of the urinary and generative organs of Verte- 

 brates arise from the mesoderm, and are closely connected with one 

 another genetically as well as morphologically and physiologically. 



I. URINARY ORGANS. 



As a rule, the development of the urinary organs from their 

 first rudiments to their final form extends through a relatively 

 long period, and the higher the animal in question, the longer this 

 period is. Unlike other organs, they do not reach their permanent 

 form step by step, without a break, but consist of a series of 

 organs, each of which has a similar function (viz., that of filtration 

 and excretion) and corresponds to a definite ontogenetic stage, 

 after which it is replaced functionally by another organ of the 

 series, and may then in part take on physiological relations to 

 the generative organs. In consequence of this peculiarity, these 

 developmental stages of the excretory organs of Vertebrates can 

 also be recognised phylogenetically, and are represented by the 

 paired organs known respectively as the fore-kidney or head- 

 kidney (pronephros), mid-kidney (mcsonephros), and hind-kidney 

 (mctanephros). 



The number of urinary organs constituting the series corres- 

 ponds in general to the position of the animal in the vertebrate 

 scale. Thus so far as is known at present, the excretory organs of 

 Amphioxus and apparently of Myxinoids correspond to a single 

 set ; in all the true Fishes and in Amphibians, there is a provi- 

 sional (pronephric) and a permanent (mesonephric) organ, while in 

 the Amniota both pronephros and mesonephros are provisional 

 and become replaced functionally by a third organ, the metane- 

 phros. 



All these organs consist essentially of epithelial canals or 

 tubules which arise from or close to that part of the mesoderm 

 which primarily connects the segmented somites with the lateral 

 plates (cf. p. 9) and is therefore often known as the nephrotomc, 

 to distinguish it from the myotome above and the lateral plates of 



