THE ORGANS OF THE MIDDLE GERM-LAYER. 



357 



from the fact that there are developed out of the wall of the body- 

 cavity, in the vicinity of the openings of its tubules, one or several 

 vascular glomeruli. In the Chick for example (fig. 201), in the region 

 from the eleventh to the fifteenth somites, there is a proliferation of 

 connective tissue on either side of the mesentery (me), by means of 

 which the right and left pronephridia are separated from each other, 

 -which grows into the body-cavity as a spheroidal body (gl). 



A blood-vessel from the aorta penetrates into each proliferation 

 and is here resolved into a tuft of capillaries, which are then united 

 again into an efferent .vessel. Only in those Vertebrates in which the 

 pronephros is functional, as in the larvae of the Amphibia, in the 

 Cyclostomes and the Teleosts, 

 does the glonierulus attain to a 

 considerable development, where- 

 as in the Selachians and Amniota 

 it remains rudimentary. In the 

 first case fluid or urine is pro- 

 bably secreted by this apparatus, 

 and then taken up by the open- 

 ings of the pronephric tubules 

 and conducted outside the body 

 by means of the pronephric duct, 

 which is to be discussed directly. 

 There is one point in this con- 

 nection that is noteworthy and 

 characteristic of the structure of 

 the pronephros : the glomerulus 

 is developed, not in the wall of 

 the pronephric tubule itself, as 

 is the case in the tubules of the mesoiiephros, but in the wall of the 

 body-cavity, so that the urine can be evacuated only through the 

 agency of the latter. 



But in what manner does the pronephros communicate with the 

 outside ? 



This communication takes place by means of a longitudinal canal, 

 which is developed in immediate continuation with the pronephros, 

 and, beginning in front, gradually grows backwards until it reaches 

 the proctoda3um and opens into the cloaca. It is found in all 

 Vertebrates in the region where the primitive segments abut upon 

 the lateral plates. At the time of its origin it is always close under 

 the epidermis, later it is farther and farther removed from the latter 



Fig. 201. Cross section through the external 

 glomerulus of a pronephric tubule of an 

 embryo Chick of about 100 hours, after 



15 U.KOUK. 



gl, Glomerulus ; ge, peritoneal epithelium ; 

 Wd, naesonephric (Wolffian) duct ; ao, 

 aorta ; me, mesentery. The pronephric 

 tubule and its connection with the glo- 

 merulus are not shown in this figure. 



