574 ON THE THEORY OF THE VERTEBRATE SKULL 



It is to Rathke's luminous researches that we are indebted for the 

 first, and indeed, even now, almost the only, demonstrative evidence- 

 of this great fact. Twenty years ago that great and laborious 

 embryologist worked out the early stages of the development of the 

 skull in each class of the Vertebrata. Confirmed and adopted by 

 Vogt and Bischoff, his conclusions have been feebly controverted, but 

 never confuted ; and my own observations lead me to believe that 

 they are destined to take a permanent place among the data of 

 biological science. Nothing is easier than to verify Rathke's views 

 in an embryonic fish or amphibian ; and as it matters not which of 

 the higher Vertebrata is selected for the study of cranial development,, 

 I will state at some length -what I have observed in the embryonic frog.'^ 



Before the dorsal laminje have united so as to enclose the primitive 

 craniospinal cavity, the anterior portion of the floor of that cavity is; 

 bent downwards. The angle which the deflexed portion forms with 

 the rest becomes less and less obtuse, until, when the dorsal laminae 

 have united and the visceral clefts have begun to appear, it constitutes, 

 a right angle. 



On examining the floor of the craniospinal cavity at this period,, 

 it is seen that the notochord, at present formed by the aggregation of 

 a number of yelk segments or embryo-cells, small in themselves, but 

 larger than those of which the rest of the body is composed, ends in a 

 point immediately behind the angular flexure. 



The notochord has no sheath as yet, and is not in any sense- 

 prolonged into the deflexed portion of the floor of the craniospinal 

 cavity. 



When the visceral clefts first appear, they are best seen from the 

 inner or pharyngeal aspect of the visceral wall. Five, of which the 

 two anterior are the longest and about equal, while the others-- 

 gradually diminish in length from before backwards, can be distinctly 

 observed. They mark out the boundaries of a corresponding number- 

 of " visceral arches,'' and there is sometimes an appearance as of a 

 sixth visceral arch behind the last cleft. A horizontal section shows, 

 that these arches differ in nothing but their relative size — in no other 

 respect can one of them be distinguished from the other. 



The anterior visceral cleft lies in a transverse plane, immediately 

 behind the angular bend of the floor of the craniospinal cavity, or, as. 

 I shall henceforward term it, mesocephalic flexure. Consequently 

 the posterior part of the first visceral arch passes into the future basis 

 cranii close to the flexure. 



The parts of the cerebrum are now distinguishable. It is bent in. 



^ See Note V. for the development of the skull in other Vertebrata. 



