122 
ME. GEOEGE W. CALLENDEE ON THE FOEMATION OF 
and rises to the front, a passage is left behind it leading from the mouth to the oesophagus, 
and common to the pharynx and to the larynx. 
At this period the membrane consists of three layers. The inner of these folds in and 
forms a septum, which divides the larynx from the common pharyngeal tube, and growing 
up below the root of the tongue forms the epiglottis. These growths are completed in 
a foetus ‘9 of an inch long. The outer layer is that in which the constrictor muscles 
originate, whilst in the anterior portion of the arch there is an intermediate or third 
layer of dense granular and subsequently cartilaginous tissue, in which are formed the 
several cartilages of the larynx, excepting the epiglottis. 
The upper portion of this arch, therefore, forms the inferior constrictor muscles, and 
at this early period of development blends with that tissue in which the middle con- 
strictor of either side is developed (Plate I. figs. 3, 4, 5). The remainder of the 
arch becomes, like the hyoid, cartilaginous in the greater part of its extent. In a foetus 
•9 the outline of the thyroid cartilage is distinct, and also that of the front and sides of 
the cricoid, but posteriorly the horns of the thyroid are imperfectly recognized, the upper 
one not being as yet cartilaginous, whilst the arytenoid and cricoid cartilages are not yet 
completely separated from the thyroid behind (Plate I. fig. 2, th, cr, ar). 
Thus far these structures whilst bent over towards the middle line are on either side 
perfectly flat, and altogether form a mass ’06 long and ‘05 wide. Their continuity with 
the pharyngeal wall is easily traced. 
In a foetus 1*2 the separation of the principal cartilages is complete and the cornua of 
the thyroid are formed (Plate I. fig. 3, th, cr, ar). Although the first granular masses 
of cartilage appear in the thyroid and in the fore part of the cricoid, yet the chondrifica- 
tion of the membrane extends so rapidly to the posterior parts of the cricoid and to the 
arytenoid, that no practical distinction can be made between the times of these quickly 
consecutive changes. 
The next step in the development of this arch consists in the curving of the cri- 
coid and arytenoid cartilages into the septum which separates the larynx from the pha- 
rynx. In a foetus 1‘4 (Plate I. fig. 4, larynx f 9 long and T wide) the turning in of 
these structures towards the middle line is evident; and in a foetus 1*8 (Plate I. fig. 5, 
larynx T2 long and T2 wide) is being completed. At this stage I notice a narrow edge 
of membrane turning down from the greater horn of the hyoid arch, and joining the 
inner and upper surface of the arytenoid cartilage (Plate I. fig. 5). The appearances 
of the fissures marking the divisions between the laryngeal cartilages are drawn in figs. 2, 
3, 4, & 5. 
The Thyroid Body. 
The external part of the lower half of the laryngeal arch developes into an accessory 
structure, the thyroid body. From the seventh to the ninth week it may be recognized as 
a distinct mass, having the characters which I have described in a previous communica- 
tion* ; it lies by the side of the arytenoid and cricoid cartilages, from which it can easily 
* Proceedings of the Eoyal Society, vol, xvi. p. 183. 
