G'JO J. TLAYFAIR MCMUKRICIT. 



Dr. Ryder refers the abnormal structure of the glU to the 

 flegeneration which has taken place, evidenced by the rudi- 

 mentary structure of the arches, and by the fact that the 

 leaflets are far less numerous than in ordinary fishes. He 

 supposes that " the reduction in number of these appendages 

 may have called for the extension of the area of the ultimate 

 branchial lamelloe or pinnae." It seems to me that this de- 

 generation of the pinnae and arches has been <lue to the almost 

 complete covering-in of the gill cavity, which is evidently an 

 old ancestral character, since, even in Stage a, the branchial 

 cavity is completely closed over by membrane. 



The dilatation of the anterior portion of the alimentary canal, 

 seen in Si age a, reminds one very strongly of the pharyngeal 

 dilation in Amphioxus, and the respiratory sac in the 

 Ascidians. 



No communication between the yolk-sac and the intestine 

 was noticeable. Von Baer states that communication exists 

 immediately behind the liver, and Lereboullet believes that 

 such a communication exists between the stomach and the 

 liver, and persists until the complete absorption of the yolk. 

 Balfour,^ however, from his observations on the Trout and the 

 Salmon, could not confirm these statements. I can agree 

 with him that " all communication between the yolk-sac and 

 the alimentary tract is completely obliterated very early." 

 Even in Stage A the intestinal wall is seen to pass quite 

 unbroken over the yolk-sac, absorption taking place through 

 the cells of the wall of the intestine, or else entirely by the 

 blood-vessels. 



Near the posterior extremity of the intestine in Stage d a 

 well-marked valve is present. In Stage A no trace of this is 

 noticeable, but it raakfs its appearance in Stage b (fig. 2), 

 where it appears as a constriction of the walls, which even- 

 tually increases, and closes off the rectum from the anterior 

 portion of the canal. 



GuELrn. June 2SUi, 1883. 



* E. M. Balfour, ' Comparal ive Embryology,' vol. ii. p. 65. 



