78 ADAM SEDGWICK. 



behind it is closely connected with the front wall of the 

 neurenteric canal ; in front it is closely connected with the 

 ectoderm of the developing buccal cavity. 



At a still earlier stage, before the cranial flexure has 

 appeared, the front end of the notochord is swollen, and runs 

 into and is continuous with the front end of the medullary 

 plate. This state of things I have myself observed at a stage 

 before the medullary plate has begun to fold. Now, my view 

 is that this connection of the notochord marks the site of the 

 future mouth ; that the site of the mouth — at first as in 

 Ascidians — perforates the medullary plate, and is on the dorsal 

 surface ; that soon, however, this site bends round on to the 

 ventral surface, and is eventually invaginated to form the 

 buccal cavity and pituitary body. This hypothesis can easily 

 be tested in the chick with the new Caldwell automatic micro- 

 tome, but I regret that I have not hitherto found time to 

 do so. 



(Professor Hubrecht in his ingenious paper already quoted 

 (this Journal, 1883), has instituted a comparison between the 

 pituitary body and notochord of Vertebrates and the proboscis 

 and proboscis sheath of Nemertines.) 



The cerebral hemispheres appear relatively late in front of 

 the notochord, and this fact fits in very well with the account 

 of their origin which I have suggested. On this view 

 Amphioxus has separated from the vertebrate stock before the 

 appearances of the cerebral hemispheres. 



The modification of the alimentary pouches, and the longi- 

 tudinal canal connecting them, I have already alluded to. It 

 only remains for me to point out that the cavities of the 

 mesoblastic somites soon come to communicate ventrally both 

 with each other and those of the opposite side; that the dorsal 

 mesentery for the most part only persists, though the ventral 

 mesentery remain in the region of the heart, liver, and behind 

 in the region of the hind part of the body ; that the nephridia 

 become modified into groups, each with a special importance; 

 the pronephros, or larval organ, is the first formed part of the 

 kidney and atrophies in the adult ; the hinder part diiferentiates 



