IN ELASMOBRANCH FISHES. 179 



summit of the spinal cord and were broadly attached to it 

 there; now their points of attachment have glided a short dis- 

 tance down the sides of the spinal cord 1 . 



The two nerve-rudiments have therefore ceased to meet 

 above the summit of the canal ; and in addition to this they 

 appear in section to narrow very much before becoming united 

 with its walls, so that their junctions with these appear in a 

 transverse section to be effected by at most one or two cells, and 

 are, comparatively speaking, very difficult to observe.. 



In an embryo but slightly older than that represented in 

 Fig. E a the first rudiment of the anterior root becomes visi- 

 ble. This appears, precisely as in Torpedo, in the form of a 

 small projection frpm the ventral corner of the ?pinal cord 

 (fig. E b, ar). 



The second step in this stage (PI. 22, fig. F) is comparable, 

 as far as the connective-tissue is concerned, with the section of 

 Torpedo (PI. 22, fig. D d). The notochord (the histological 

 details of whose structure are not inserted in this figure) is 

 rather more developed, and the segmental duct, as was the case 

 with the corresponding Torpedo -embryo, has become hollow at 

 its anterior extremity. 



The embryo from which the section was taken possessed five 

 visceral clefts, but no trace of external gills. 



In the section represented, though from a posterior part of 

 the body, the dorsal nerve-rudiments have become considerably 

 larger than in the last embryo ; they now extend beyond the 

 base of the neural canal. They are surrounded to a great ex- 

 tent by mesoblastic tissue, which, as in the case of the Torpedo, 

 takes its origin from two sources, (i) from the commencing 

 vertebral bodies, (2) from the summits of the muscle-plates. 



It is in many cases very difficult, especially with chromic- 

 acid specimens, to determine with certainty the limits of the 

 rudiments of the posterior root. 



1 [May 18, 1876. Observations I have recently made upon the development of 

 the cranial nerves incline me to adopt an explanation of the change which takes place 

 in the point of attachment of the spinal nerves to the cord differing from that enun- 

 ciated in the text. I look upon this change as being apparent rather than real, and 

 as due to a growth of the roof of the neural canal in the median dorsal line, which 

 tends to separate the roots of the two sides more and more, and cause them to assume 

 a more ventral position.] 



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