EXCRETORY ORGANS. 691 



makes its appearance in the column (fig. 386, sd}. The knob 

 forms the only structure which can be regarded as a rudtmeflt of 

 the pronephros. 



spa 



FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL 



CLEFTS. 



The sections illustrate the development of the segmental duct (pd) or primitive 

 duct of the pronephros. In A (the anterior of the two sections) this appears as a 

 solid knob (pd) projecting towards the epiblast. In B is seen a section of the column 

 which has grown backwards from the knob in A. 



spn. rudiment of a spinal nerve ; me. medullary canal ; ch. notochord ; X. sub- 

 notochordal rod ; tup. muscle-plate ; mp' . specially developed portion of muscle-plate ; 

 ao. dorsal aorta; pd. segmental duct; so. somatopleure ; sp. splanchnopleure ; pp. 

 body cavity ; ep. epiblast ; al. alimentary canal. 



While the lumen is gradually being formed, the segmental 

 tubes of the mesonephros become established. They appear to 

 arise as differentiations of the parts of the primitive lateral plates 

 of mesoblast, placed between the dorsal end of the body cavity 

 and the muscle-plate (fig. 386, si) 1 , which are usually known as 

 the intermediate cell-masses. 



The lumen of the segmental tubes, though at first very small, 

 soon becomes of a considerable size. It appears to be established 

 in the position of the section of the body cavity in the inter- 

 mediate cell-mass, which at first unites the part of the body 

 cavity in the muscle-plates with the permanent body cavity. 

 The lumen of each tube opens at its lower end into the dorsal 

 part of the body cavity (fig. 386, sf), and each tube curls obliquely 



1 In my original account of the development I held these tubes to be invaginations 

 of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of 

 my original statement from his investigations on the chick ; and from a re-examina- 

 tion of my specimens he arrived at the results stated above, and which I am now 

 myself inclined to adopt. 



442 



