DEVELOPMENT. 



66T 



more are formed in front of this point; and the series is continued back- 

 ward till the whole medullary canal is flanked by them (Fig. 449). That 

 which is first formed corresponds to the second cervical vertebra. From 

 these somites the vertebrae and the trunk muscles are derived. 



Head and Tail Folds. Body Cavity. Every vertebrate animal 

 consists essentially of a longitudinal axis (vertebral column) with a 

 neural canal above it, and a body-cavity (containing the alimentary 

 canal) beneath. 



We have seen how the earliest rudiments of the central axis and the 

 neural canal are formed; we must now consider how the general body- 

 cavity is developed. In the earliest stages the embryo lies flat on the 



FIG. 452. Diagrammatic section showing the relation in a mammal between the primitive alimen- 

 tary canal and the membranes of the ovum. The stage represented in this diagram corresponds to 

 that of the fifteenth or seventeenth day in the human embryo, previous to the expansion of the 

 allantois; c, the villous chorion ; a, theamnion; a', the place of convergence of the amnionand 

 reflexion of the false amnion, a" a", or outer or corneous layer; e, the head and trunk of the 

 embryo, comprising the primitive vertebrae and cerebro-spinal axis; t, i, the simple alimentary 

 canal in its upper and lower portions. Immediately beneath the right hand i is seen the foetal 

 heart, lying in the anterior part of the pleuro-peritoneal cavity; v, the yolk-sac or umbilical vesicle; 

 vi, the vitello-intestinal opening; u, the allantois connected by a pedicle with the anal portion of 

 the alimentary canal (Quaim. 



surface of the yelk, and is not clearly marked off from the rest of t\i& 

 blastoderm: but gradually the head-fold or cresceiitic depression (with 

 its concavity backwards) is formed in the blastoderm, limiting the head 

 of the embryo; the blastoderm is, as it were, tucked in under the head, 

 which thus comes to project above the general surface of the membrane:, 

 a similar tucking in of blastoderm takes place at the caudal extremity, 

 and thus the head and tail folds are formed (Fig. 452). 



Similar depressions mark off the embryo laterally, until it is com- 



