1040 EMBRYOLOGY. 



The integument covering the pharyngeal bones is sometimes imprisoned at 

 some points, when fusion of the bones of these arches occurs. There then result 

 small dermoid cysts that slowly enlarge, and which the surgeon is sometimes 

 called upon to remove from time to time, as they disfigure the face — if they have 

 no other inconvenience. 



C. Development of the Thorax. — The ribs are dependencies of the protoverte- 

 bral laminte, which curve towards the lower face of the vertebral column. The 

 true ribs are most rapidly developed, and before attaining the middle line are 

 united by their internal extremity, and form a moiety of the sternum. A fissure 

 separates the costal arches of the right side from those of the left : this gradu- 

 ally contracts, and finally disappears, and the sternum is then formed. The ribs 

 are, after the petrous bone, the parts of the skeleton which are most promptly 

 ossified, ossification commencing in the middle ribs. 



The costal arches do not belong exclusively to the dorsal vertebrae, but have 

 a tendency to form along the whole length of the spine ; and it is not rare to 

 see— attached to the lumbar vertebrae— a small cartilaginous nucleus, which is 

 soon lost in the texture of the abdominal walls. This nucleus assumes large 

 dimensions on the last cervical vertebrae of Birds. 



The form of the thorax varies with the species ; in some it is circular, in 

 others it is flattened laterally ; and in all cases it is less developed in the foetus 

 and young animal than in the adult. It is in the latter that the thoracic cavity 

 presents, proportionately, its greatest dimensions. 



D. Development of the Limbs. — The limbs do not show themselves until after 

 the formation of the spinal column, the pharyngeal arches, and the thoracic 

 parietes. They appear as four little prolongations from the thorax and pelvi^ 

 and are slightly enlarged at their origin and constricted in the middle. Their 

 free extremity is flattened, and either divides or remains single, as the animal 

 has one or more apparent digits. It is in these prolongations that the carti- 

 laginous segments are developed, which, at a later period, become the bones of the 

 limbs. For the manner in which ossification is carried on in each bone, reference 

 must be made to Osteology, Articles IV. and V., pp. 97 and 127. 



2. Muscles. — The muscles are developed around the bones when these have 

 become perfectly distinct. They may be divided into four groups — the vertebral 

 muscles, which come from the muscular laminae of the protovertebrae ; the 

 visceral muscles — thoracic and abdominal cavities, neck and jaw — having the 

 same origin ; the cutaneous muscles, which are developed at the expense of 

 the cutaneous laminae of the middle layer of the blastoderm ; and the muscles 

 of the hmbs, the development of which is not yet perfectly known. 



It was at one time believed that the muscular fibres were formed by the 

 junction — end to end — of several elongated cells ; but it is now known that 

 they are constituted by a single cell which lengthens, and the nuclei of which 

 multiply and lie at the surface, while its contents are transformed into a substance 

 that offers the characteristics of contractile tissue. The sarcolemma is formed 

 after the fibre, by a modification of the connective tissue surrounding it. 



Development of the Circulatory Apparatus. 



During the first days which follow the appearance of the embryo in the 

 substance of the blastodermic layer, there is no trace of vessels in the area 

 germinativa. It is not long, however, before the heart and some blood-vessels 



