222 ESSENTIALS OF ZOOLOGY 



structures of this system arise from the middle portion (mesomere) 

 of the mesoderm germ layer. This portion of the mesoderm de- 

 velops as a fold of tissue just lateral to the midline on each side, 

 extending from the thorax to the pelvis and projecting down into 

 the body cavity slightly. The earliest and most anterior part of this 

 fold is the pronephros. In higher vertebrates this is strictly an em- 

 bryonic structure, though it does develop some Malpighian corpuscles 

 which exist for a time. The pronephric duct which develops and 

 leads from the pronephros to the cloaca persists, later becoming the 

 Wolffian duct. Next follows the mesonepliros, which is more elaborate 

 and really functions as a kidney for a time in the embryo of reptiles, 

 birds, and mammals, and throughout the life of fishes and amphibia. 

 It partially overlaps the posterior portion of the pronephros and ex- 

 tends several segments farther posteriorly. The collecting tubules, 

 which receive urine from the Malpighian corpuscles, reach the surface 

 of the mesonephros and join the pronephric duct leading to the cloaca. 



In the reptiles, birds, and mammals, the permanent kidney arises 

 from still a third development, the ynetanephros. This is formed at 

 the free end of the metanephric duct which is an outgrowth of the 

 pronephric duct near the cloaca. The metanephros overlaps the pos- 

 terior portion of the mesonephros. There are numerous Malpighian 

 corpuscles in the tissue of this body, and the metanephric duct be- 

 comes the ureter. The Wolffian duct becomes the vas deferens of 

 the male but degenerates in the female except for a vestige. The 

 urinary bladder develops as a ventral do^vnpushing from the cloaca. 



A genital ridge develops along the ventral margin of each meso- 

 nephros before it degenerates, and from this fold a gonad arises. 

 Regardless of future sex, there develops also in each individual, a 

 Miillerian duct from the ventrolateral portion of the mesonephric 

 body. This duct becomes the functional oviduct of the female, but 

 it degenerates in the male except in animals like the leopard frog. 



The ovaries develop in their permanent location, but the testes of 

 most male mammals are derived from the mesonephros in the dorsal 

 part of the body cavity and at a certain age descend through an 

 inguinal canal to the scrotum which is outside the body Avail. 



The external genital organs develop as general structures with no 

 distinction as to sex until about the end of the second month in the 

 human being. At that time the penis takes form from the phallus 

 or genital tubercle, if the individual is to be male. In the female 



