Fossit agnaths In the cephalaspids a fossa in the rear wall 

 of the head skeleton is thought to have been formed by a 

 pronephric mass. 



Genera/ observations Comparisons of the lamprey and 

 myxinid kidneys are difficult. They agree in having the kid- 

 ney extending through a large number of body segments; 

 this is related probably to their eel-like form. They agree in 

 having the nephric duct formed by segmental contribution 

 rather than by growing back from the pronephric area. 

 They agree in lacking "mesonephric" peritoneal funnels. 



They differ, however, in many ways. The pronephros of 

 the lamprey is never a blood-cell forming area, while the 

 opisthonephros contains much hemopoietic tissue. The pro- 

 nephros of the myxinid appears to function largely in blood- 

 cell production and phagocytosis and is not connected with 

 the excretory part of the kidney. The opisthonephros of the 

 lamprey is quite complex in its structure with a suprarenal 

 sinus separated from the excretory-lymphoid tissues of the 

 kidney below and the postcardinal vein above. The glomeru- 

 lar band of the lamprey is a unique feature. The opistho- 

 nephros of the myxinid is simple, consisting of segmental 

 tubules with glomeruli. The nephric duct is similar histologi- 

 cally to the tubule and apparently functions in athrocytosis 

 (phagocytosis of certain large molecules), resorption, and 

 secretion. The blood supply of the tubule has not been de- 

 scribed, but probably there are small channels in the con- 

 nective tissue sheath which are supplied by the renal arter- 

 ies and drained by the renal veins. 



The development of the kidney of the myxinid is unique 

 and it is the simplest of the vertebrates. There is superficial 

 agreement with the shark in that funnels form in all seg- 

 ments. However, the funnels are not the same, being pro- 

 nephric throughout in the myxinid and peritoneal in the 

 mesonephric region of the shark. The myxinid is peculiar in 

 that an entire nephrotome is consumed in forming a defini- 

 tive tubule and a part of the nephric duct. The simplicity of 

 this kidney is probably related to the marine habitat and is 

 not an indication of primitiveness. 



The kidney of the lamprey is also specialized since fun- 

 nels, of any kind, are not formed in the mesonephric region. 

 The multiplication of glomeruli and the great hemopoietic 

 development also suggests modification. 



General observations on the excretory system 



A clear picture of the evolution of the vertebrate excre- 

 tory system is still lacking. It is usually suggested that the 

 primitive system was a holonephros, with one or more tu- 

 bules per body segment, extending the length of the body 

 cavity. This view has some justification; however, there are 

 other alternatives. It is quite possible that the original kid- 

 ney was essentially a pronephros, each tubule functioning in 

 the recovery of materials from the coelomic fluid as well as 

 from the glomerular filtrate. The early appearance of this 



anterior region in ontogeny preadapted this kidney to serve 

 the larvae of more advanced vertebrates, while progressively 

 more posterior parts of the nephrogenic cord, still largely or 

 entirely undifferentiated, developed into more elaborate 

 and complex kidneys adequate for a larger and more active 

 organism. Another view would be that the pronephric kid- 

 ney is a larval specialization. It developed progressively 

 more anteriorly, as a reponse to the advantage of the earlier 

 differentiation of that region, with the result that it was able 

 to satisfy the needs of larval forms with longer and longer 

 periods of growth and differentiation. Differentiation in- 

 cluded the production of a more complex kidney in the 

 primitive kidney region situated well back from the pro- 

 nephros. 



The excretory system of Amphioxus as well as the general 

 story of kidney ontogeny supports the first view, while the 

 fact that the functional kidneys of all living vertebrates lie 

 well back in the body cavity, separated by a gap from the 

 pronephric region, gives some support to the latter. The 

 holonephros theory appears to be largely a compromise sup- 

 ported by the developmental stages of some sharks and the 

 myxinoids, types which do not seem to be particularly 

 primitive in terms of their kidney structure. 



THE REPRODUCTIVE SYSTEM 



Mammals 



The male The male reproductive system consists of testis, 

 efferent ducts, modified tubules of the kidney, the nephric 

 duct — now called the vas deferens — and accessory glands 

 developed from the vas deferens (Figure 10-31 ). 



The testis is a rounded mass whose interior is filled with lo- 

 bules of looping and interconnected seminiferous tubules 

 (Figure 10-32). Each lobule is encapsulated, along with the 

 whole testis, in connective tissue, the tunica albuginea. The 

 outer surface of the testis is smooth, without indication of 

 the internal lobules. Several seminiferous tubule loops unite 

 and enter a marginal network of tubules, the central canal 

 or rete testis, by way of a tubulus rectus or straight tubule. 

 Connecting the rete testis with the embryonic mesonephric 

 kidney is a series of efferent tubules which pass through the 

 supporting mesentery, the mesorchium, and enter the re- 

 mains of a part of the mesonephric kidney. The modified 

 kidney tubules, or tubuli mesonephroi, along with a part 

 of the nephric duct, the ductus epididymidis, form the epidid- 

 ymis. The vas deferens leading back from the epididymis 

 differs from its precursor, the nephric duct, in having an in- 

 vestment of smooth musculature. The vas deferens leads into 

 the urogenital sinus, or urethra, through an enlarged area, 

 the ampulla. Above or anterior to the entrance of the vas 

 deferens into the urethra is the bladder. Also at this junction 

 are the seminal vesicles and the prostate gland. The urethra 

 distally passes through an intromittent organ, the penis. 

 Bilateral bulbourethral or Cowper's glands enter the ure- 

 thra behind the erectile tissue of the penis. 



318 -THE UROGENITAL SYSTEM 



