VERTEBRATE ANIMALS 217 



myosepta. The haemal processes of the anterior trunk vertebrae are 

 reduced to mere ribless stumps, while in the caudal region they are 

 greatly expanded and pass off more ventrally as flattened plates to 

 curve inwards and meet in the middle line. They are completed by 

 a series of mid- ventral haemal spines, which are the bases for the 

 support of the ventral portions of the caudal fin. In this way 

 another tube, the haemal canal, is formed beneath the centra, 

 similar to the neural canal above them, and in life this contains the 

 caudal artery and caudal vein, the main vessels of the tail. 



Turning now to consider the development of the vertebral 

 column, we find that the first part of the skeleton to make its appear- 

 ance in the embryo is a structure known as the notochord or chorda 

 dorsalis, which arises as a rod of cells derived from the dorsal part 

 of the entoderm of the gut. It is, therefore, ventral to the central 

 nervous system under which it stretches from the posterior end to 

 just below the fore-brain, where it becomes thin and runs off into a 

 downwardly turned end. The cells, disc-shaped to start with, soon 

 secrete around themselves a clear refractive elastic membrane, the 

 so-called primary chordal sheath or membrana elastica externa. 

 The cells enlarge and become vacuolated by the formation within 

 them of a jelly-like substance, and so exhibit in section, a character- 

 istic appearance producing the typical notochordal tissue. The 

 vacuoles fill the inside of the cells, and so reduce the cytoplasm 

 to an enveloping layer. At first scattered, the nuclei migrate to 

 the periphery of the chorda, there forming a layer termed the chorda 

 epithelium. This soon secretes a second thicker and more fibrous 

 layer within the former, and termed the secondary chordal sheath 

 or membrana elastica interna. The mesoderm (mesenchyme) 

 surrounding the chorda produces migrant cells which wander into 

 the inner sheath and gradually transform it into a thick cellular 

 layer, now termed the tunica skeletogena. 



From this a large part of the centrum is formed ; the neural 

 arches, etc., are laid down in an extension of the mesenchyme layer 

 which encloses the notochord and passes up to surround the spinal 

 cord, and is called the skeletogenous sheath. In this are differ- 

 entiated four longitudinal bands of deeper staining more tightly 

 packed cells, two dorso-lateral and two ventro-lateral. Paired 

 cartilages appear in the bands, and on the dorsal side are the 

 beginnings of the vertebral and intervertebral neural plates, and on 

 the ventral side the haemal arches. Vertical rings of cartilage are 

 now laid down in the tunica skeletogena corresponding in position 

 with the arches, and these constitute the primordia of the centra. 

 As they grow they become much thicker in their central region, thus 

 constricting the notochord intracentrally, but they always leave a 



