THE DBVELOPMElirT OF MOTELLA MUSTELA. 303 



scarcely be said to be anterior to the fore brain ; but, as will be 

 seen from figures 9 and 10, it occupies a position quite as promi- 

 nent. Tbe truth is, that the brain develops very rapidly, while 

 the jaws remain comparatively undeveloped, so that the fore brain 

 is pushed forwards and downwards until the mid brain lies com- 

 pletely over it. Another consequence of this rapid growth is 

 that the mouth takes up a temporary ventral position (figs. 9 & 

 10). At this time the mouth is only slightly open, and the lower 

 jaw is not so prominent as the upper one. Their relative position 

 will easily be understood by reference to fig. 9 a. When, how- 

 ever, the brain has reached its maximum development, the ventral 

 portion of the head begins to play its part, and the cranial axis 

 is gradually pushed back into its normal position as the lower 

 jaw increases in size. This time the greatest development is on 

 the extreme ventral surface, and the lower jaw now outstrips the 

 upper one in dimensions. The time occupied by all these changes 

 is from 6 to 6g days, and their course will easily be followed by 

 a comparison of figs. 7 to 14. 



About a day after hatching, a small clear vesicle arises in the 

 immediate vicinity of the liver, which is probably the gall- 

 bladder. This vesicle remained transparent as long as the young 

 fish lived. In Cyclopterus Iv/mpus, however, I have observed a 

 similar gland, which gradually becomes filled with a bright green 

 fluid, seen at a glance in the living embryo. As is usual with 

 pelagic fish-eggs, there is no circulation either embryonic or 

 vitelline in Motella before hatching, nor, indeed, for some days 

 afterwards. From six to seven days after hatching a rudimentary 

 circulation was observed, but it was very faint, and no vessels 

 were properly formed. Soon afterwards the young fish died, so 

 that I have no reliable data on this point. The arrangement of 

 the vessels in the early circulation of Teleostean embryos seems 

 to vary very much in different forms that have been studied; and 

 so far as I am aware no thorough comparative study of this de- 

 velopment has yet been made. In Oyclopterus I have found an 

 arrangement differing in many important points from other forms 

 already described, and Motella seems to have something in com- 

 mon with Cyclopterus. There is, first of all, an aortic circula- 

 tion pushed backwards as far as the anus, and the corpuscles 

 return in a lower area of the same tissue before the vessels them- 

 selves are formed. Erom this system a branch is given off near 

 the constriction in the intestine, which soon bifurcates, sending a 

 branch backwards to the anal gut, and a branch forwards to the 



