DEVELOPMENT. 



777 



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. 



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

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

 which clearly defines it from the yolk. 



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

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



Fig. 470. Diagrammatic section showing the relation in a mammal between the primitive 

 alimentary canal and the membranes of the ovum. The stage represented in this diagram cor- 

 responds to that of the fifteenth or seventeenth day in the human embryo, previous to the ex- 

 pansion of the allantois ; c, the villous chorion ; a, the amnion ; a', the place of convergence of 

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

 trunk of the embryo, comprising the primative vertebrae and cerebro-spinal axis; t, t, 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 ; v i, the vitello-intestinal opening ; w, the allantois connected by a pedicle 

 ^ith the anal portion of the alimentary canal. (Quain.) 



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 yolk, corresponding to the well or 

 undecked portion of the canoe. 



This communication between the embryo and the yolk is gradually 

 contracted by the further tucking in of the blastoderm from all sides, 

 till it becomes narrowed down, as by an invisible constricting band, to 



