i in: ORG UJfl F Tin: MIIUM.K ,i:i:\i LAI ER. 



357 



o 



Ix. 



from tl.e t';..-t that t here are developed out of tin- wall of the body- 

 cavity, in the vicinity of tin* opening of !<> niiml. >. one or se 



ular ^lomeriili. In tin- ( 'hi -k for example (tig. 'J<>1 ), in t he region 

 from the eleventh to the lift eent h M>mite>, t here is & proliferation of 

 eonnecti\e tissue on either side of the mi by mea; 



which the right ami left prom-phridia arc separated fr< 111 each other, 

 which grows into tin- 1-ndy cavity as a spheroidal body (<//). 

 A blood-vessel from the aorta penetrates into each proliferation 



and is here iv>olved into a tut't of capillaries. \\liich are tlien united 

 airain into an eil'erent v >>.!. Only in those Vertebrates in which the 

 pi-one] hms is functional, as in the lar\;e of tlie Amphibia, in the 

 Cyclostoin,-> and the Telf 

 does the ;lnnenilMs attain to a 

 considerable development, where- 

 a> in the Selachians and Anmiota 

 it remains rudimentary. In the 

 first caM> Huid or urine is pro- 

 l>a 1 >ly 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 discued directly. 

 There is one point in this con- 

 nection that is noteworthy and 

 characteristic of the structure of 

 the pronephros : the glomerulus 

 is de\elo}td, not in the wall of 

 the pronephric tubule itself, as 

 is the case in the tubules of the meson, phros, hut in the wall of the 

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

 ;u-y of the latter. 



But in what manner does the pronephros communicate with the 

 outside? 



Tins communication takes place by mean > of a longitudinal canal, 

 which is developed in immediate continuation with the pronephros, 

 and, he^i lining in front, gradually Arrows hackwards until it reaches 

 the proctodaeum 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 lection through the external 

 glomerulus of a pronephric tubule of an 

 embryo Chick of about 100 hours, after 



KM . 



inenilus; ;/ . fjuthelium; 



uiesouephric (\V,.ltfi:m) duct; ao, 



The I'l.'iLfphric 



tubule aii'l it^ o . m, ,v: ion \\itli the glo- 

 iiH-rulus are not shown in this tigure. 



