440 INTRODUCTION TO CRANIATA [CH. 



to be made up by the fusion of the outer portion of successive kidney 

 tubules. The kidney in Vertebrates is usually said to be divided into 

 an anterior, a middle and a posterior region termed pronephros, 

 mesonephros, and metanephros respectively, but these three 

 portions are of very different values. The pronephros consists of 

 two or three, rarely five or six tubules, which are outgrowths from 

 nephrotomes which lie immediately below the region of the gills for 

 which reason the pronephros is sometimes termed the head-kidney. 

 In each nephrotome there is formed a protrusion from its inner 

 wall which abuts on the dorsal aorta. The protrusion takes 

 the form of a plug containing a plexus of blood-vessels projecting 

 into the cavity of the nephrotome. From this cavity the blood is 

 only separated by a thin layer of epithelium, so that water can 

 pass by diffusion from the blood into the nephrotome. This plug 

 is termed the glomerulus. These anterior nephrotomes fuse with 

 one another so as to form one cavity on each side and this cavity, 

 the so-called " pronephric chamber," sometimes becomes completely 

 shut off from the splanchnocoel (as in Fish), sometimes (as in 

 Amphibia) its opening into the splanchnocoel enlarges so that it 

 ceases to be distinguishable from the latter space. The pronephros 

 only persists during the early larval condition of the animal. As 

 the animal grows it entirely disappears. It is to be looked on as 

 a precocious development of the most anterior tubules of the kidney 

 to serve the needs of larval life. 



The middle portion of the kidney is called the meson ephros. 

 In this region the nephrotomes remain separated from one another 

 and constitute what are known as the Malpighian capsules of 

 the kidney, and into each of these capsules a glomerular plug pro- 

 jects. In the lower Craniata (Elasmobranch, Fishes, Amphibia) the 

 nephrotomes retain their connection with the splanchnocoel by 

 long necks which become ciliated and are known as ciliated 

 peritoneal funnels, but in all the higher Craniata the nephro- 

 tomes become definitely shut off from the splanchnocoel. The 

 nephrotome or Malpighian capsule can bud off a similar smaller 

 capsule which in turn produces a kidney tubule, the so-called 

 secondary tubule. The tubule produced by the original nephro- 

 tome is called the primary tubule. The secondary tubule opens 

 into the longitudinal duct which becomes enlarged so as to receive 

 primary and secondary tubules. The longitudinal duct thus be- 

 comes somewhat moniliform, i.e. with alternate swollen and 

 narrow places (C, Fig. 216). In the hinder region of the kidney or 



