288 MR. G. BROOK ON THE 



point wliere the apex is hidden in ordinary views behind the 

 head. 



The areas where the embryonal pectoral fins are to develop are 

 at first granular patches situated on each side about the level 

 where the diverticulum of the liver arises. Within the area of 

 each patch, a little later, a longitudinal ridge-like thickening of 

 the epiblast is pushed out, which gradually extends into a fin. 

 The ventral fins are formed in a similar manner somewhat later. 

 The position of these fins, at first parallel to the notochord, after- 

 wards at a considerable angle to it, will best be seen from the 

 figures. After hatching, the bases of both pairs of fins are drawn 

 down ventrally, and as the yolk-sac becomes absorbed, the fins 

 lie closer to the body. 



No detailed observations have been made on the development 

 of the excretory organs and genital ducts. The vesicle near the 

 anus which develops into the urinary bladder (u.v.') was first 

 noticed on the seventh day. Its appearance at time of hatching 

 is shown in fig. 27, with the commencement of the Wolffian duct 

 (w.d.) leading from it. 



The young fish generally begin to hatch out on the tenth day ; 

 somewhat accelerated individuals and batches on the ninth, and 

 slightly retarded ones not till the eleventh day. Young fish of 

 the same batch would continue to hatch out at intervals for two 

 or three days afterwards. The young fish, for some hours after 

 hatching, lies on its side, or more often quite on its back, but be- 

 gins to right itself as the large yolk-sac is absorbed. The length 

 of the newly hatched embryo is 3"5 millim. The mouth, which is 

 only indicated by a slight depression in the newly hatched em- 

 bryo, is well formed, and the jaws have a slight motion 24 hours 

 afterwards, and by this time also the yolk-sac has become entirely 

 absorbed. 



Although I have been able to keep some specimens a week after 

 hatching, further development was slight and probably abnormal, 

 as I never succeeded in feeding any of the young fish. 



Summaty. 



The egg of TracJiinus is about 1'32 millim. in diameter, and 

 contains from 20 to 30 small oil-globules, thus differing from the 

 majority of floating fish eggs hitherto described. 



In the unfertilized egg a vitelline membrane is easily distin- 



