534 THE DEVELOPMENT OF THE INDIVIDUAL 



Cardiac muscle develops only in the heart from the splanchnic 

 mesoderm which forms the main layer of the developing auricles and 

 ventricles. This muscle becomes striated, and each fiber connects with 

 other fibers, thus forming a continuous network. 



Urogenital System. The kidneys originate in a pair of longitudinal 

 folds of mesoderm which form from the nephrotome or intermediate 

 plate of mesoderm. From this plate there arises a series of tubules 

 which at first open into the coelom by ciliated funnels. Actually, there 

 are three different kidneys formed in the evolution of vertebrates, and 

 all of these are formed in the embryos of higher vertebrates. In the 

 frog the head kidney or pronephros arises first in the anterior end of 

 the coelom and functions for several weeks in the frog tadpole. It is 

 followed by the mesonephros or midkidney which gradually enlarges 

 and takes over the excretory function when the tadpole metamorphoses 

 and forms a frog. At this time the pronephros degenerates and dis- 

 appears. In higher animals including man, the hind kidney or meta- 

 nephros is formed from the posterior end of the nephrotome and re- 

 places the mesonephros. The latter does not disappear completely, but 

 leaves some important vestiges behind in the male, namely the epi- 

 didymis and vasa efferentia. 



The gonads arise as genital ridges on the dorsal side of the body 

 cavity, apparently from the coelomic epithelium. Since their origin is 

 exactly the same in the male as in the female, the sex of an embryo can- 

 not at first be determined. In this genital ridge there soon appear 

 large round cells, the primitive germ cells. The oviduct appears in 

 both sexes as a fold of the coelomic epithelium and is quite separate 

 from the sperm duct. The latter develops from the mesonephric duct 

 which at first carries only urine. In mammals the gonads gradually 

 differentiate into ovaries or testes. If testes develop they then migrate 

 posteriorly. In man the first difference in the sexes is not visible until 

 the end of the seventh week. A pair of pockets from the coelom forms 

 in most mammals at the posterior end of the body into which the testes 

 descend shortly before birth — the ovaries remain in the body cavity. 



The external genital organs also follow parallel lines of develop- 

 ment with similar structures being developed in both sexes until the end 

 of the second month in the human embryo. At first there is present only 

 a urogenital slit surrounded by the labioscrotal folds and a genital 

 tubercle at the anterior end. As development proceeds in the male, the 

 genital tubercle greatly enlarges and forms the penis and the folds 

 close to form the scrotum. In the female, the tubercle becomes the 

 clitoris and the folds form the labia, or lips of the urogenital region. 



