98 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



The manner of development of these parts in the Dog-fish 

 is different from anything that previous investigators have met 

 with in other classes, but I believe that it gives a clearer insight 

 into the true constitution of these parts than vertebrate embryo- 

 logy has hitherto supplied, and at the same time renders easier 

 the task of understanding the differences in the modes of de- 

 velopment in the different classes. 



I shall commence with a simple description of the observed 

 facts, and then give my view as to their meaning. At about 

 the time of the appearance of the third visceral cleft, and a 

 short way behind the point up to which the alimentary canal 

 is closed in front, the splanchnopleure and somatopleure fuse 

 together opposite the level of the dorsal aorta. 



From the mass of cells formed by this fusion a solid knob 

 rises up towards the epiblast (PI. 4, fig. 1 1 b, o v], and from 

 this knob a solid rod of cells grows backwards towards the 

 tail (fig. \\c, o v} very closely applied to the epiblast. This 

 description will be rendered clear by referring to figs, lib and c. 

 Fig. lib is a section at the level of the knob, and fig. 1 1 c is a 

 section of the same embryo a short way behind this point. So 

 closely does the rod of cells apply itself to the epiblast that 

 it might very easily be supposed to be derived from it. Such, 

 indeed, was at first my view till I cut a section passing through 

 the knob. In order, however, to avoid all possibility of mistake 

 I made sections of a large number of embryos of about the age 

 at which this appears, and invariably found the large knob in 

 front, and from it the solid string growing backwards. 



This string is the commencement of the Oviduct or Mil Her' s 

 duct, which in the Dog-fish as in the Batrachians is the first 

 portion of the genito-urinary system to appear, and is in the 

 Dog-fish undoubtedly at first solid. All my specimens have 

 been hardened with osmic acid, and with specimens hardened 

 with this reagent it is quite easy to detect even the very smallest 

 hole in a mass of cells. 



As a solid string or rod of cells the Oviduct remains for 

 some time ; it grows, indeed, rapidly in length, the extreme hind 

 end of the rod being very small and the front end continuing 

 to remain attached to the knob. The knob, however, travels 

 inwards and approaches nearer and nearer to the true pleuro- 



