THE FCETUS. 911 



the sixth or seventh month they cover the body. They may be shed and 

 renewed before birth. They are developed in a prolongation of the epi- 

 dermic lamina, which is imbedded in the substance of the derm ; it is 

 (shaped like a little bottle, and is composed of a mass of cells ; in its centre, 

 these cells are modified and collected together to form a small cone, whose 

 base covers the growing papilla. This cone becomes elongated, touches the 

 surface of the epidermis, doubles under the effort to push through it, and 

 finally makes its exit, after which it can grow freely. 



The sebaceous and sudoriparous glands are developed in the same manner, 

 towards the middle period of uterine existence. 



" The horny productions the claws, hoofs, ergots, cJiesnuts begin to 

 show themselves early. Towards the end of the second month, in the 

 foetus of the Cow, there is perceived, at the extremity of each limb, a 

 small, pale, translucid, conical tubercle, which is the rudiment of the hoof. 

 At the commencement of the fourth month, 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 until birth. In Solipeds, the " chesnuts " 

 are shown at mid-term in the shape of thin brown plates, which are soon 

 darker-coloured." l 



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 LOCOMOTOBY APPARATUS. 



1. THE SKELETON. We have seen at page 16 how the development and 

 growth of the bones takes 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 VERTEBRAL COLUMN. The vertebral column is 

 the first portion of the skeleton manifested in the embryo ; it is represented 

 by the chorda dorsalis, 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 becoming developed, these parts encircle the latter 

 and tho medullary canal ; from this results the external tlieca 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 vertebrae, 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 immediately behind the preceding and the intervertebral disc. 



The bodies of the vertebrae are developed more rapidly than their spinal 

 portion : thus, towards the end of the second month, all the vertebral bodies 

 are already cartilaginous, while the vertebral laminae are yet membranous. 



1 Colin. * Traite de Physiologie Comparee des Animaiix.' 2nd Edition, Paris, 1873. 



