INTERNAL ANATOMY. 



6 9 



my 



These are the ccelomic openings, or nephrostomes, of the 

 tubules. At their outer ends most of them open directly 

 into a longitudinal duct, \kspronephric duct, which extends 

 backwards to the cloaca. 

 The most anterior tubules, 

 however, tend to fuse to- 

 o-ether at their outer ex- 



O 



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 



pig 23. Schematic transverse section 

 If nOW we inquire into through a Selachian embryo in the region 



of the pronephros. (After VAN WlJHE.) 



the mode of development The dotted line drawn across the section 

 ,- . , indicates the plane of division between the 



of such a pronephnc sys- upper segme F nted and the lower unseg- 



tem as the One above de- rnented portions of the primitive body-cavity 



(procoelom). my. Myotome or myomere. 



Scribed, we find that its . Mesomere or nephrotome. p. Prone- 

 , , phric outgrowth, sp. Unsegmented bodv- 



component tubules arise as avity or - splanchn ; coel , sdero.ome. 



a Series of knob-like Seg- Nerve-tube, ch. Notochord. ao. Dor- 

 sal aorta, a/. Digestive tube. 



mental outgrowths from 



the outer or somatic layer of the peritoneum at the base 



of the segmented portion of the primitive body-cavity. 



