128 



SEGMENTAL INVOLUTIONS. 



Fig. 4. — Two sections of a Pristiurus Embryo with three visceral clefts. 



spn 



The sections are to shew the development of the segmental duct {{'pd) or primi- 

 tive duct of the kidneys. In A (the anterior of the two sections) this 

 appears as 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. 



spn. rudiment of a spinal nerve ; mc. 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. splanchnopleura ; 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 Plate X. fig. 7sd. 



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 (PL x. 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 rod the cells are still rounded. In no part of it 

 has a lumen appeared. 



At this period also the knob, partly owing to the com- 

 mencing separation of the muscle-plate from the remainder of 

 the mesoblast, begins to pass inwards and approach the pleuro- 

 peritoneal cavity. 



At the same stage the first not very distinct traces of the 

 remainder of the urinary system become developed. These 



