KIDNEYS 87 



early stage. They form more or less solid masses, which may fuse 

 into a continuous strand on either side. Subsequently they 

 become hollowed out, forming coiled tubules with Malpighian 

 capsules (urocoeles) and glomeruli, and generally coelomostomes 

 (mesonephric funnels) (Fig. 55, B). The main canal of the tubule 

 grows outwards towards the longitudinal duct of the pronephros, 

 with which it fuses. This duct is now partly or entirely converted 

 into a mesonephric duct (Fig. 56, B). If the nephrotome has become 

 completely separated off from the lateral plate, the opening to the 

 coelom may be reaccpiired ; but as a rule it persists throughout the 

 development of the tubule. At first the mesonephric tubules are 

 strictly segmental ; in Myxinoids only do they remain so (p. 50). In 

 other Craniates, at all events throughout the greater length of the 

 mesonephros, a varying number of new tubules are formed from 

 masses of cells nipped off' from the first rudiment. All the 

 mesonephric tubules are thei'efore derived from the same original 

 series of rudiments by a sort of budding. These secondary tubules 

 acquire the typical structure and relations, excepting for the 

 coelomic funnel which is not developed. The tubules become 

 crowded, and their metameric order is lost. The coelomostomes 

 survive only in adult Elasmobranchs and Amphibia (Bles [36]). 

 The mesonephric tubules are developed from the anterior to the 

 posterior end of the abdominal cavity, but some tubules in front 

 and behind may degenerate. 



In some fish (Elasmobranchii) and in Amphibia the posterior 

 region of the mesonephros is much more developed than the anterior, 

 and functions as the chief excretory organ. But in the Amniota the 

 mesonephros disappears almost entirely in the later stages of 

 development, and is replaced behind by the permanent kidney or 

 metanephros. In essential structure the metanephros resembles 

 the mesonephros, but it never has any coelomostomes, shows no 

 obvious trace of segmental order, and possesses its own duct, the 

 ureter. Some authors believe the metanephros to be entirely 

 formed by an outgrowth from the hinder end of the meso- 

 nephric duct (Minot, Schultze, etc.) ; but others have shown 

 (Sedgwick, Schreiner, Felix [135], etc.) that only the ureter 

 and the collecting tubes are so developed, the excretory tubules 

 and capsules being derived from a separate rudiment. The 

 metanephric rudiment is in the shape of a mass of cells continuous 

 with the mesonephric rudiments in front, and derived like these 

 from the intermediate cell-mass, but of more posterior segments. 

 Coelomostomes are here permanently lost. 



Much controversy has taken place with regard to the 

 morphological significance of the three divisions of the excretory 

 organ. One of three view r s may be held : they are merely three 

 regions of a once continuous kidney, which have come to function, 



