346 



DEVELOPMENT OF ELASMOBRANCH FISHES. 



derived from that layer were it not for the sections shewing its 

 origin from the knob above mentioned. We have in this rod the 

 commencement of what I have elsewhere 1 called the segmental 

 duct. 



FlG. 4. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL CLEFTS. 



A B 



spa 



spn. 



The sections are to shew the development of the segmental duct (pd) or primitive 

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

 knob projecting towards the epiblast. In B is seen a section of the column which 

 has grown backwards from the knob in A. 



spit, rudiment of a spinal nerve ; me. medullary canal ; ch. notochord ; X, string 

 of cells below the notochord ; mp. muscle-plate ; mp' . specially developed portion of 

 muscle-plate ; ao. dorsal aorta ; pd. segmental duct ; so. somatopleura ; sp. splanch- 

 nopleura ; pp. pleuroperitoneal or body-cavity ; ep. epiblast ; al. alimentary canal. 



My observations shew that the segmental duct is developed 

 in the way just described in both Pristiurus and Torpedo. Its 

 origin in Pristiurus is shewn in the adjoining woodcut, and in 

 Torpedo in PI. 1 1, fig. 7, sd. 



At a stage somewhat older than I, the condition of the 

 segmental duct has not very materially altered. It has in- 

 creased considerably in length, and the knob at its front end 

 is both absolutely smaller, and also consists of fewer cells than 

 before (PI. 1 1, fig. 7, sd]. These cells have become more columnar, 

 and have begun to arrange themselves radially ; thus indicating 

 the early appearance of the lumen of the duct. The cells forming 

 the front part of the rod, as well as those of the knob, commence 

 to exhibit a columnar character, but in the hinder part of the 



1 " Urinogenital Organs of Vertebrates," Jonrn. of Anat. and Phys. Vol. x. 

 [This Edition, No. vn.] 



