INTERiVAL ANA TOA/Y. 



69 



These are the coelomic openings, or nephrostomes, of the 

 tubules. At their outer ends most of them open directly 

 into a longitudinal duct, the pronep/inc duct, which extends 

 backwards to the cloaca. 

 The most anterior tubules, 

 however, tend to fuse to- 

 gether at their outer ex- 

 tremities, before reaching 

 the common duct. Corre- 

 sponding to each tubule 

 there is a short artery- 

 growing out from the dor- 

 sal aorta, and abutting with 

 its blind end against the 

 portion of the body-cavity 

 into which the innermost 

 nephrostomes open. 



Later on these coecal 

 outgrowths from the dorsal 

 aorta develop a vascular 

 network at their free ends, 

 and so produce a series of 

 glomeruli. 



If, now, we inquire into 

 the mode of development 

 of such a pronephric sys- 

 tem as the one above de- 

 scribed, we find that its 

 component tubules arise as 

 a series of knob-like seg- 

 mental outgrowths from 

 the outer or somatic layer of the peritoneum at the base 

 of the segmented portion of the primitive body-cavity. 



Fig. 33. — Schematic transverse section 

 through a Selachian embryo in the region 

 of the pronephros. (After VAN WlJHE.) 



The dotted Hne drawn across the section 

 indicates the plane of division between the 

 upper segmented and the lower unseg- 

 mented portions of the primitive body-cavity 

 (proccelom). my. Myotome or myomere. 

 nis. Mesomere or nephrotome. /. Prone- 

 phric outgrowth, sp. Unsegmented bod\'- 

 cavitv or splanchnoccel. sc. Sclerotome. 

 n. Nerve-tube. .://. Notochord. ao. Dor- 

 sal aorta, al. Digestive tube. 



