124 
ME. GEORGE W. CALLENDER ON THE EORMATION OF 
the integumental covering. If this fold is everted, a band of tissue can be traced to an 
angle above, where it comes into contact with the well-defined tissue in which the larynx 
is to be formed (Plate I. fig. 6). On the inner surface of the fold two ridges (Plate I. 
fig. 6, c , s ) are imperfectly marked, but at the lower end of the membrane they are more 
distinct, — one ending in a rounded border (and here the clavicle is developed), the other, 
which lies behind the first, descending a little lower and being slightly bent with the 
convexity of the curve backwards (in this the scapula is formed). 
In a foetus more advanced ('55), after removal of the integument and of the upper 
thoracic extremity, the plate of membrane is more clearly defined. It grows from the 
occipital region, and above is in close relationship with the membrane for the laryngeal 
arch (Plate I. fig. 7). Traced downwards, it sends forward a comparatively delicate 
layer of tissue (Plate I. fig. 7, c), which extends to the superior extremity of that (st) 
in which the sternum and the costal cartilages are being formed. At about the middle 
of the lowest border of the arch a rounded mass, which is granular, and in which carti- 
lage is appearing, forms a considerable elevation (s), pushed forward as it is by the ribs 
which have now grown up beneath it (r). This rod is curved, as shown in fig. 7 ; its 
superior and anterior extremity is the acromion, its inferior extremity is the coracoid 
process; and nearly midway between the two a bud-like outgrowth (which seems to 
originate in a folding over of the integument), the thoracic extremity, has been removed 
in the specimen from which the drawing (fig. 7) was made. 
The changes thus far related consist in the division of the membrane growing out 
from the occipital region into two portions, the anterior of which developes into the 
hyoid and laryngeal arches, whilst the posterior division descends and forms a broad 
layer in the neck (covered by integument), which eventually forms the sterno-mastoid 
and trapezius muscles of either side, and behind joins above in the middle line the tissue 
from the opposite side at the ligamentum nuchae : below it sends out two processes ; one 
of these grows forward to form the clavicle and to join the sternum, the other curves 
downwards and forms the scapula, and from this there grows out the thoracic extremity. 
It will be noticed that these outgrowths, including the lingual, correspond not inex- 
actly with the course of the cerebral nerves distributed below the mandible, — (1) the hypo- 
glossal, (2) the glossopharyngeal, (3) the pneumogastric, and (4) the spinal accessory. 
In a foetus ’9 long the clavicle has ossified in its middle third, the remainder is carti- 
laginous ; it is ‘09 of an inch long. When the ossification of this bone is described as 
taking place in membrane, it is to be understood that only the earliest formed portion 
of the bone is so ossified ; the greater part of the clavicle, about two-thirds of its length, 
ossifies in cartilage, which rapidly takes the place of the original membrane, and of 
which only small plates are left at either end of the bone in a foetus T2 (in Plate I. 
figs. 8 and 9, the earliest membrane ossification and the extension of the ossification in 
cartilage are shown). 
By the growth of the sternum to the middle line and by the attachment to it of the 
