356 F. M. BALFOUR. 



time renders easier the task of understanding the differences 

 in the modes of development 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 (PL XV, fig. 11 b, o v), and from 

 this knob a solid rod of cells grows backwards towards the 

 tail (fig. 11 c, v) very closely applied to the epiblast. This 

 description will be rendered clear by referring to figs. 11 b 

 and c. Fig. 11 6 is a section at the level of the knob, and 

 fig. 11 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 Mutter'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 length of the rod being very small and the front end 

 continuing to remain attached to the knob. The knob, how- 

 ever, travels inwards and approaches nearer and nearer to the 

 true pleuro-peritoneal cavity, always remaining attached to 

 the intermediate cell mass. 



At about the time when five visceral clefts are present the 

 Oviduct first begins to get a lumen and to open at its front 

 end into the pleuro-peritoneal cavity. The cells of the rod 

 are first of all arranged in an irregular manner, but gradu- 

 ally become columnar and acquire a radiating arrangement 

 around a central point. At this point, where the ends of all 

 the cells meet, a very small hole appears, which gradually 

 grows larger and becomes the cavity of the duct (fig. 12, ov). 

 The hole first makes its appearance at the anterior end of the 



