GENERATION AND DEVELOPMENT. 



259 



the latter adheres more or less closely to the hypoblast, and gives rise to 

 the serous and muscular walls of the alimentary canal and several other 

 parts (Fig. 414). 



The united parietal layer of the mesoblast with the epiblast is termed 

 Somatopleure, the united visceral layer and hypoblast, Splanchnopleure. 



FIG. 414. Transverse section through dorsal region of embryo chick (45 hours). One half of the 

 section is represented: if completed it would extend as far to the left as to the right of the line of the 

 medullary canal (Me). A, epiblast; C, hypoblast, consisting of a single layer of flattened cells; Me, 

 medullary canal : Pv, protovertebrse ; Wd, Wolffian duct; /So, somatopleure ; Sp, splanchnopleure; 



pp, pleuro-peritoneal cavity; c/i, notochord; ao, dorsal aorta, containing blood-cells; v. blood-vessels 

 of the yolk-sac. (Foster and ~ 



Balfour.) 



The space between them is the pleuro-peritoneal cavity, which becomes 

 subdivided by subsequent partitions into pericardium, pleura, and peri- 

 toneum. 



Head and Tail Folds. Body Cavity. Every vertebrate animal 

 consist 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 sur- 

 face of the yelk, and is not clearly marked off from the rest of the blas- 

 toderm: but gradually a crescentic depression (with its concavity backward) 

 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. 415). 



Similar depressions mark off the embryo laterally, until it is completely 

 surrounded by a sort of moat which it overhangs on all sides, and which 

 clearly defines it from the yelk. 



This moat runs in further and further all round beneath the over- 

 hanging embryo, till the latter comes to resemble a canoe turned upside- 

 down, the ends and middle being, as it were, decked in by the folding or 

 tucking in of the blastoderm, while on the ventral surface there is still a 

 large communication with the yelk, corresponding to the "well" or un- 

 decked portion of the canoe. 



