DEVELOPMENT OF PETROMTZON FLUVIATILIS. 347 



there is no bar in front of the first persistent cleft. They are 

 slightly curved inwards towards the median linein the middle part 

 of their course where they bend round the external opening of 

 the cleft. About the thirtieth day they fuse with one another 

 ventrally and so two rods are formed which lie close together 

 in the posterior half of their course but diverge round the 

 thyroid. About the same time each bar sends forward two 

 processes, one above and the other below the opening of the 

 gill to vrhich it belongs ; these ultimately fuse with the pos- 

 terior edge of the gill bar next in front. The processes of the 

 most anterior bar fuse with each other. Dorsally the last six of 

 the bars also become continuous (fig. 42), and form two longi- 

 tudinal bars which run parallel and close to the notochord. 

 The most anterior bar does not join this rod but sends a process 

 inwards, serving to support the auditory capsule, which lies 

 just in front of it directly over the first persistent gill-cleft. 



The first traces of the basi-cranial skeleton appear on the 

 thirtieth day as two rods of cartilage, the trabeculse (figs. 40). 

 They lie close against the notochord for their posterior two 

 thirds, anteriorly, however, they diverge and snrround the 

 pituitary space. About six days after their first appearance 

 the trabeculse send out laterally a transverse bar of cartilage 

 which passes out on each side in front of the auditory capsule, 

 lying between the ganglia of the fifth and seventh nerves. Pro- 

 fessor Parker has identified this as the rudiments of the pedicle 

 and pterygoid. They lie in the tissue of the bar which is in 

 front of the first gill-cleft which has long ago disappeared. 



Immediately beneath the trabeculse the carotid artery runs 

 forward as an anterior continuation of the dorsal aorta. The 

 trabeculse have become continuous with the dorsal end of the 

 most anterior branchial bar, which is not united with the longi- 

 tudinal bar formed from the fused dorsal end of the other six. 

 The connection is very slight but is quite evident in sections, 

 between this and the dorsal end of the second bar some little 

 space exists, the latter when it commences lies at a slightly 

 lower level than the trabeculse. 



The above description represents the condition in my oldest 



