DIFFERENTIATION OF SOMITES 505 



Theoretically, it appears that the waste matter containing nitrogen 

 which is elaborated in the primitive liver and collected in the coelom, 

 together with the coelomic fluid itself, passes outward through the 

 nephrostomes and tubules in each segment. In higher forms all the 

 parts are more differentiated and some of the segmentation is lost. 



Figure 168 gives a clear understanding of the earthworm's segmented 

 excretory system which represents the pronephridic type of kidney. 



Such a primitive type of nephridia, if completely developed, may be 

 described as follows : At the proximal end of the tubule, a ciliated fun- 

 nel, the nephrostome, opens into the coelom. The cilia may continue 

 into the tubule to produce a current which will carry the coelomic fluid 

 into and through the tubule. The tubule expands into a Malpighian or 

 renal corpuscle. This corpuscle consists of a vesicle known as Bow- 

 man's capsule, one side of which projects into the other, so that the 

 cavity is nearly filled. The inturned portion is the glomerulus, consist- 

 ing of a network of capillary blood vessels, supplied by an artery and 

 drained by a vein. Beyond the Malpighian corpuscle the tubule becomes 

 convoluted, while its cells become glandular. The first convoluted tubule 

 is followed by a straightened portion forming a simple U shape. The 

 arms of the U form the ascending and descending limbs. The entire U 

 is called Henle's loop. Then follows a second convoluted tubule which 

 passes by means of a short connecting tubule into the non-glandular 

 collecting tubule. Other groups of similar-formed excretory units enter 

 this same collecting tubule, which then leads into a urinary duct through 

 which the waste matter is carried out of the body. 



Various parts of the complete system just described may be miss- 

 ing in different groups of animals. For example, in Amniotes, the 

 nephrostomes are never formed, though they are in Ichthyopsida. 



In the pronephros, the Malpighian corpuscle is quite rudimentary 

 and often entirely lacking, and there is also no differentiation into con- 

 voluted tubules and Henle's loop. 



The renal corpuscles form a sort of filtering apparatus by which 

 water is passed from the blood-vessels of the glomerulus into the tubules 

 near their beginning, which liquid thus serves to carry out the urea, uric 

 acid, etc., which has been secreted by the glandular portions of the walls 

 of the tubules. 



A varying number of nephrotomes form in different animal forms, 

 and so also a varying number of nephrostomes form. Figure 300 will 

 give the student a general idea of how mesonephros, and metanephros, 

 follow each other and just what their relations are. 



The tiny tubules must not, however, be confused with the ducts. 

 The ducts represent the collecting tubule described above. 



The pronephric tubules grow first and then join the, pronephric 

 ducts. Later the mesonephric tubules grow caudad to the pronephric 

 tubules and join the same ducts. The original pronephric tubules then 



