420 



minal are the same in each ; the posterior crus of the suborhitar 

 arch answers to the suspensorium of Teleostei, its anterior crus to 

 their palatopterygoid apparatus. But with all this, there are dis- 

 crepancies in the structure of the skull itself, which would forbid too 

 close "an approximation between the bony and the unossified crania, if 

 their adult forms alone were examined. The study of the development 

 of the ossified vertebrate skull, however, eliminates this difficulty, 

 and satisfactorily proves that the adult crania of the lower Vertebrata 

 are but special developments of conditions through which the em- 

 bryonic crania of the highest members of the subkingdom pass. 



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 em- 

 bryologist 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 develop- 

 ment, I will state at some length what I have observed in the em- 

 bryonic frog.* 



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

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

 is bent downwafds. 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 pro- 

 longed into the deflexed portion of the floor of the craniospinal cavity. 



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



* See Note V. for the development of the skull in other Fertebrata. 



