302 MB. &. BROOK ON SOME POINTS IN 



embryo of Trachinus (Linn. See. Journ. Zool. vol. xviii. pi. 6. 

 figs. 27 and 28), shows at once that the Motella embryo is not so 

 advanced in development as that of Trachinus. This of course 

 might be expected from the earlier period at which it hatches. 

 The head is strangely backward in development in the newly- 

 hatched fish, and, as we shall see, presents very curious features 

 during its later modifications. The thickening for the ventral 

 fins is only just visible, as will be seen in fig. 7. It is about 

 this time that the air-bladder is budded off" from the respiratory 

 section of the alimentary tract just in front of the liver. It 

 does not, however, become very distinct until the body becomes 

 more thickly pigmented, when it is easily seen as a clear space 

 surrounded by pigment. The embryo in this stage measures 

 2*25 miUim. long by '55 millim. deep. At the time of hatching 

 there is little pigment on the body, but this is quickly deposited, 

 until when two days old there is so much pigment about the 

 jaws and mesenteron as to make further development difficult to 

 follow. Figures 8 and 8a give ventral and lateral views of the 

 embryo, about forty hours later than fig. 7. The yolk-sac has 

 been considerably absorbed, leaving the heart in a large peri- 

 cardiac cavity, the remains of the segmentation-cavity, which 

 persists so long as any yolk is left. The ventral fins have grown 

 considerably, and are now nearly as large as the pectoral. The 

 principal changes, however, are in the head. The brain-lobes 

 have increased very considerably in size, are well marked out 

 for the first time on a lateral view, the cerebellum has now been 

 segmented oif, and the medulla oblongata is very much increased 

 in bulk. Changes now take place in the head which are difficult 

 to follow ; and further work on this point is necessary in order to 

 understand them properly. The dorsal portion of the head grows 

 much more rapidly than the ventral portion up to a certain point. 

 The result is that a cranial flexure is produced which is difi'erent 

 from anything I have observed in other Teleosteans, and is in 

 some respects comparable with the early embryonic condition of 

 Elasmobranchs. In the latter, it is true that the characteristic 

 feature of this flexure is that the mid brain is pushed forward 

 until it forms the most prominent part of the body, and is in 

 fact, for the time being, the termination of the body-axis. In 

 Elasmobranchs, however, the mouth is situated ventrally in the 

 adult, so that the embryonic mouth and brain have not again to 

 change their relative position, as would be the case in a Teleostean 

 with an embryonic cranial flexure. In Motella the mid brain can 



