192 ON THE MORPHOLOGICAL CONSTITUTION OF 



Tlio allocation of tlie mandibular haemal arch to the post- 

 sphenoidal or first post-stomal sclerotome follows from the 

 analysis already made of the pre-stomal sclerotomes. 



The Second and Tliird Fost-stomal Hcemal Arches. — As the 

 researches, more especially of Eathke and Eeichert, on the 

 development of the first visceral lamina, afford a clue to the 

 constitution of the corresponding haemal arch, so do the 

 labours of these observers in like manner indicate the nature 

 of the second and third arches. These arches are developed 

 in the second and tliird visceral laminae, and, from the varied 

 forms which they present in the series, could only have been 

 determined by an appeal to embryology. 



In the mammal the primordial cartilaginous streak in the 

 second visceral lamina, and which is attached superiorly to 

 the auditory region, divides into segments, the uppermost of 

 which becomes the stapes ; while the succeeding become, in 

 succession with the intermediate soft portions, the " stapedius 

 muscle," the pyramid and its prolongation downwards, the 

 styloid process, the stylo-hyoid ligament, and the series of 

 sclerous elements which terminates below in the anterior 

 horn of the hyoid. 



The primordial cartilaginous streak in the tliird visceral 

 lamina is attached to the occipital region, breaking up into 

 four segments ; the two upper disappear ; the two lower be- 

 come respectively the posterior horn and corresponding half 

 of the body of the hyoid. 



In the second visceral lamina of the bird, in like manner, 

 the auditory columella is developed superiorly, and the feeble 

 anterior horn of the hyoid below, while the elements of the 

 suspensory or posterior horn of the hyoid are formed in the 

 third visceral lamina. The fibrous septum of the tongue and 

 the epiglottis of the mammal make their appearance in the 

 line of junction of the second and third visceral laminae. The 

 respective share taken by these two laminae in the formation 



