1038 



EMBRYOLOGY. 



the fourth montli, or thereabouts, the hoof, better defined, has become firm and 

 opaque, and has assumed a fine yellow tint. At mid-term, brown or black 

 patches are manifest if the coronet is provided with pigment ; it is only about 

 the end of gestation that the hoof towards the coronet begins to have the 

 greenish hue peculiar to horn destitute of pigment, but the remainder of this 

 production — especially at the inferior part — preserves its yellow colour uutil birth. 

 In Solipeds, the ' chestnuts ' are shown at mid-term, in the shape of thin brown 

 plates, which are soon darker-coloured." * 



The horn of the hoof is not at first tubular ; after birth it is shed, and is 

 succeeded by a more consistent tubular horn, which had been forming beneath it. 



Development of the Locomotory Apparatus. 



1. The Skeleton. — ^We have seen at p. 19 how the development and growth 

 of the bones take place ; it is therefore needless to recur to this subject here ; so 

 we will limit ourselves to an examination of the mode of development of the 

 principal sections of the skeleton. 



A. Development of the vertebra. — The vertebral spine is the first portion of 

 the skeleton manifested in the embryo ; it is represented by the chorda dorsalis 



Fig. 572. 



Fig. 573. 



DIAGRAM SHOWING THE POSITION OF THE 

 CHORDA DORSAUS IN THE BODY OF THE 

 VERTEBRA, AND THE FORMATION OF THE 

 DIAGRAM SHOWING THE ATTENUATION OF NEURAL ARCHES. 



THE CHORDA DORSALIS IN THE MIDDLE OF 



THE BODIES OF THE VERTEBRA, WHILST c^> Chorda dorsalis ; cv, body of the verte- 



PRESERViNG ITS ORIGINAL DIAMETER IN bia ; a, ueural arch, or neurapophysis ; c, 



THE INTERVERTEBRAL SPACES. "b ; ^w, transverse process. 



ch, Chorda dorsalis; v, body of vertebra ; U, 

 intervertebral spaces. 



— a stalk constituted by a mass of cells situated in the interior of a transparent 

 sheath. The protovertebrce. appear on each side of the chorda dorsalis ; in becom- 

 ing developed, these parts encircle the latter and the medullary canal ; from this 

 results the external theca of the cord, and the superior uniting membrane. From 

 this time, the vertebral column exists in the shape of a membranous axis. 



Soon this membranous spine is segmented to give rise to the vertebrae, and 

 its various portions gradually become cartilaginous. Each persistent vertebra 

 does not exactly correspond to a protovertebra. In reality, the latter takes a 

 share in the formation of two vertebrge, and divides into two portions — an 

 inferior, which constitutes the posterior moiety of a permanent vertebra ; and 

 a superior, which forms the anterior moiety of the persistent vertebra imme- 

 diately behind the preceding and the intervertebral disc. 



' Colin, Trait€de Fhysiologie Compar^e des Animaux, 2nd Edition. Paris, 1873. 



