THE SKELETON OF THE VERTEBRATE HEAD. 187 



arches are similar. Firstly, all the visceral lamiiue in which 

 they are developed appear to consist of the serous, vascular, 

 and mucous layers united ; secondly, the cartilaginous streaks 

 are formed towards their inner surfaces, under the mucous 

 layer ; thirdly, the heai-t and vascular arches are on their ex- 

 terior, under the serous layer. 



First Post-stumal Ho:mal Arch. — The constitution of this 

 arch must be determined by the examination of the develop- 

 ment of the first post-stomal visceral lamina. It has been 

 already stated that the process usually considered as the upper 

 part of the so-called " first visceral lamina " is, if its general re- 

 lations be taken into accoimt, the posterior pre-stomal visceral 

 lamina in which the pre-spheuoidal hiumal arch is developed. 



The cartilaginous streak in the first post-stomal visceral 

 lamina of the mammal divides into two portions. The 

 superior and smaller of the two becomes the incus. The long 

 inferior portion is the cartilage of Meckel ; around the lower 

 part of which the corresponding half of the lower jaw is de- 

 veloped ; the upper part forms the slender process and the 

 head of the malleus. As the Eustachian tube, the tympanum, 

 and the external auditory passage, consist of the persistent 

 upper portion of the first visceral cleft, the cartilage of the 

 Eustacliian tube, and the tympanic bone, which are continuous 

 with one another, and form the floor of these three spaces, are 

 developed in blastema deposited near the upper extremity of 

 the cleft. This blastema also forms the membrane of the 

 tympanum, into which the handle of the malleus shouts. It 

 is to be obsei"ved that this lower jaw and tympanic bone do 

 not originate in the primordial cartilaginous streak, but in 

 blastema deposited around it. Tlie tympanic bone fonns at 

 first an inverted arch across the viscend cleft, or a ring in- 

 complete above, which supports the membrane of the tym- 

 panum on the outer side of the attenuated i)ortion of Meckel's 

 cartilage, which connects the malleus to the inner side of the 



