648 BASHFORD DEAN, 



Larva ofthe fourth day (PI. 9, Fig. 10; dorsal aspect 

 PI. 9, Fig. 11; ventral aspect PL 9, Fig. 12). 



In this larva there is an evident reduction in the size of the 

 yolk sac, the latter's hinder end has now become pointed, its ventral 

 side concave; anteriorly it is overlapped by the growth of the gular 

 isthmus of the opercular flaps; these are now seen to open and shut, 

 showing that the active process of breathing has begun ; gill filaments 

 are established, but as yet are small in size. The chin region is now 

 prominent ; conical teeth appear ; the iris Coming to be formed, shows 

 on the ventral side a well marked break due to the connection of its 

 growth with the choroidal fissure; the heart has completed its 

 S-shaped course. The thickening of the ectodermal tracts of the 

 head surface is now to be prominently noted ; and the pushing of the 

 nerve tract of the lateral line has progressed along the trunk region 

 into the anterior part of the tail. The outline of the larval caudal flu 

 is at this stage relatively largest; the pectorals, much enlarged in 

 size, indicate that the fin axis has come to protrude from the body 

 wall, and that it is already surrounded by the rapidly growing dermal 

 margin of the fin. At this and earlier stages the pectorals are 

 situated ou the yolk sac close by the side of the trunk, their planes 

 diverging somewhat tailward. In the ventral view of this larva, 

 PI. 9, Fig. 12, a remarkable arrangement of pigment may be seen 

 in the chin and sucking disc, demarking in this region apparently the 

 lines of the sensory tracts; especially striking is a cross line of pig- 

 ment traversing both halves of the sucking disc. 



Larvaofthefifthday (PI. 9, Fig. 13 ; dorsal view PI. 9, Fig. 14). 



A notable decrease in the size of the yolk sac characterizes this 

 stage also; the sac's hinder end is now reduced to a pointed out- 

 growth, now no further from the body than the rim of the anal fin- 

 fold. The size of the larva has on the other band notably increased; 

 its head is especially prominent; the mandible is now distinct, oc- 

 casionally opening and closing; the nasal sac is prominent, its open- 

 ing somewhat pear-shaped, — at its narrow end near the eye will be 

 constricted off the posterior naris; the iris is now metallic in lustre, 

 marked here and there with dark pigment. The mucous canal tracts 

 are to be clearly followed on the head surface; the lateral line has 

 passed into the region of the tail tip and has turned ventrally, as 

 Allis has shown, into the position of the median line of the defini- 

 tive caudal fin. The continued growth of the opercular folds has by this 



