412 THE EVOLUTION OF MAN. 



are only physiologically comparable; they are, therefore, 

 analogous, as having the same function; morphologically, 

 however, they are not to be compared, or are not homo- 

 logous. 188 The false urinary bladder in Fishes is a pro- 

 duct of the primitive kidney duct, therefore of the skin- 

 layer ; the true urinary bladder in Dipneusta, Amphi- 

 bia, and Amniota is, on the contrary, a blind-sac of the 

 terminal intestine, and hence a product of the intestinal 

 layer. 



In all low Skulled Animals (Craniota), without amnion 

 (in Cyclostoma, Fishes, Dipneusta, and Amphibia), the 

 urinary organs remain in an inferior stage of development, 

 in so far as the primitive kidneys (protonephra), though 

 much modified, here act permanently as urine-secreting 

 glands. In the three higher vertebrate classes, included in 

 the term Amnion Animals, on the contrary, this is the case 

 only for a short period during early embryonic life. The 

 permanent, or secondary kidneys (renes, or metanephra), 

 which are peculiar to these three classes, are very early 

 developed. These originate, not (as was long believed, on 

 the authority of Remak) as entirely new, independent 

 glands of the intestinal tube, but from the posterior section 

 of the primitive kidney duct (protwreter). From the latter, 

 near where it opens into the cloaca, a simple pouch — the 

 secondary kidney duct — grows out, and this increases con- 

 siderably in length forwards; from the blind, upper, or 

 anterior portion of this the permanent kidney originates, 

 precisely as the primitive kidney originates from the pri- 

 mitive kidney duct. The secondary kidney duct gives rise 

 to a number of small blind tubes — the secondary urinary 

 tubes — and the blind capsule-shaped ends of these 



