VII.] THE VISCERAL ARCHES. 179 



From the study of the development of the skull, especially in some of tlie 

 lower vertebrates, Mr Parker and Professor Huxley have shewn, that the 

 trabeculae are developed independently of tlie investing mass, and that their 

 subsequent connection with it is due to a secondary process. Professor Huxley 

 is of opinion that they are to be regarded as the remains of a pair of visceral 

 arches, corresponding with the other live pairs of arches which we find developed 

 ill the chick. The stage in which they exist as simple visceral arches with a 

 core of undifferentiated mesoblast is not seen in the chick. They first attract 

 notice when they become cartilaginous rods. 



The ordinary visceral arclies are, as we have seen, suffici- 

 ently obvious, while as yet their mesoblast is quite unditfer- 

 entiated ; but in them, as in the trabeculse, rods of cartilage 

 are subsequently developed and begin to make their appear- 

 ance about the fifth day. 



The first arch, it will be remembered, budded off a 

 process called the superior maxillary process. The whole 

 arch, therefore, comes to consist of two parts, viz. a superior 

 and an inferior maxillary process; in each of these, carti- 

 laginous rods are developed. In the superior maxillary 

 process, the rod does not appear till the fifth day. It is 

 called from its subsequent fate, the ptery go-palatine rod, and 

 consists of a pterygoid and of a palatine part. In the 

 inferior maxillary process two developments of cartilage take 

 place ; one which forms the quadrate in the upper or prox- 

 imal portion close to the origin of the superior maxillary 

 process, a second in the lower or distal portion, which goes 

 by the name of Meckel's cartilage. 



Cartilaginous rods are also formed in the second and third 

 arches. These, which give rise to the hyoids and branchials 

 respectively, quickly come to lie within the first arch, but 

 do not form a conspicuous portion of the skeleton of the 

 face. 



4. Closely connected with the development of the skull 

 is the formation of the parts of the face. 



After the appearance of the nasal grooves, on the fourth 

 day the mouth (Fig. 56 M.) appears as a deep depression 

 inclosed by five processes. Its lower border is entirely 

 formed by the two inferior maxillary processes (Fig. 56, F.l), 

 at its sides lie the two superior maxillary processes S. M, 

 while above it is bounded by the fronto-nasal process nf. 



After a while the outer angles of the fronto-nasal process, 

 enclosing the termination of the ethmovomerine plate, pro- 

 ject somewhat outwards on each side, giving the end of the 



12-2 



