14 



Mr. Parker on the Structure and 



[Jan. 10, 



Comparing" my own observations on this low type with the resnlts 

 given of the study of the chick in Foster and Balfour's excellent work, 

 I find that few paragraphs in it would need any material alteration, 

 and that the figures would mostly serve very accurately if in that work 

 the word chick were to be exchanged for that of snake embryo. 



The development of the vesicles of the brain, the organs of special 

 sense, the rudiments of the cranium and face — those things that come 

 across my path, to say nothing of the rest of the growing germ, all are 

 developed similarly in the snake, below, and in the bird, above. 



Like all the Vertebrata above the fishes and amphibia (" Ichthy- 

 opsida "), the embryo of the snake has the folds and clefts behind the 

 mouth few and small; yet there is no change of pattern, and the 

 rudiments of three pairs of gill- arches with their clefts can be made 

 out. 



But no embryo I have studied shows more clearly that all the folds 

 that lie about the mouth, before and behind, as well as those that form 

 the mouth itself, belong to one category. 



Whilst the brain is merely a row of three vesicles, and the rudiments 

 of the sense- capsules are three folds, or unclosed vesicles, the sides of 

 the head grow downwards as a series of folds parted by deep notches 

 or clefts. 



These clefts are the " naso-laerymal " in front of the rudimentary 

 mouth ; the mouth-clefts (right and left) ; the " tympano-eustachian ; " 

 and the three " hyo- branchial " clefts behind that. 



The first folds lie under the fore-brain; these are the " fronto-nasal 

 processes " or flaps ; the second, are the " maxillo- palatine ; " the third, 

 the mandibular; the fourth, the hyoid ; and the fifth, sixth, and 

 seventh are rudimentary branchials, that never give off branchial fila- 

 ments or plicae. 



The cavity of the mouth is formed by the absorption of the meso- 

 blast that lay in the angle of the head — bent double upon itself — just 

 below the end of the notochord. 



The walls of the mouth are formed by the maxillo-palatine rudi- 

 ments in front, and the mandibular rudiments behind ; below, the floor 

 of the face is suppressed ; at the sides, the right and left cleft form the 

 right and left angle of the mouth. 



The visceral folds develop but little cartilage ; an outgrowth from 

 the anterior ends of the trabeculse grows downwards and backwards 

 into the fronto-nasal process. It represents two visceral rudiments, 

 the axis of the pre-maxillary arch ; but this flap is single. 



The maxillopalatine folds develop no cartilage, but become rich in 

 "parosteal " bones. 



Each mandible develops a large quadrate and a long articulo- 

 meckelian cartilage. 



All the cartilage seen on each side behind the lower jaw is a small 



