406 EMBRYOLOGY. 



8. From the primitive segments of the head are formed the muscles 

 of the eyes, and from the visceral-arch cords the masticatory muscles, 

 the muscles of the hyoid arch and also those of the small bones of the 

 ear (1). 



The Development of the Urogenital System. 



9. The first fundament of the urogenital system is the same in 

 both sexes: it consists of (1) three pairs of canals the mesonephric 

 duct, the Miillerian duct, and the ureter; (2) four pairs of glands 

 the pro-, meso-, and metanephros and the sexual gland, which at first 

 is indifferent. 



10. The mesonephric duct arises in its most anterior part out of 

 a groove-like evagination or a ridge-like thickening of the parietal 

 middle layer ; posteriorly it detaches itself from its parental tissues, 

 fuses with the neighboring outer germ-layer, and thereby forms at 

 first a short, tubular communication between the ccelom and the 

 surface of the body. 



11. The mesonephric duct is gradually converted into a long 

 canal, inasmuch as it grows backward on the outer germ-layer, 

 which forms a thickened ridge, until it opens out into the cloaca 

 (terminal part of the hind intestine). 



12. The pronephros (head-kidney) is developed at the anterior 

 part of the mesonephric duct in the following manner : the duct, 

 upon being constricted off from the parietal middle layer, remains in 

 connection with the latter at several places, and the resulting cords 

 of connection grow out into long pronephric tubules, at the inner 

 openings of which an intraperitoneal vascular glomerulus is estab- 

 lished out of the wall of the body-cavity. 



13. Behind the pronephros the mesonephros (primitive kidney) 

 arises thus : when the primitive segments are constricted off from 

 the lateral plates, segmentally arranged cellular tubes or cords 

 (nephrotome) are formed, which communicate at one of their ends 

 with the body-cavity and at their other ends put themselves into 

 connection with the laterally situated mesonephric duct and become 

 the mesonephric tubules. (Development of Malpighian corpuscles, 

 of secondary and tertiary mesonephric tubules and the glomeration, 

 of the latter.) 



14. In the higher Vertebrates the development of the primitive- 

 kidney is to a certain extent abbreviated, in so far as the separate 

 cords of cells which arise at the constricting off of the primitive 

 segments lie very close together and constitute an apparently 



