1 68 ELEMENTARY BIOLOGY. [CHAP. 



a. The neural plate; a thickened medio-dorsal 

 sheet of epiblast, partly invaginated and en- 

 closing a deep neural groove; its raised edges 

 {neural folds). 



/3. The arckenleron ; a wide cavity situated nearer 



the upper than the lower pole, 

 y. The hypoblast; a single layer of cells forming 



the roof and side walls of /?: its cells, somewhat 



flattened above, graduating into the lower layer 



cells laterally. 



S. The notochord; a median oval mass of small 

 cells, immediately beneath the neural plate, 

 not yet completely constricted off from the 

 hypoblast. 



€. The mesoblast; interposed between the epiblast, 

 and hypoblast with the lower layer cells; most 

 marked dorso-laterally and symmetrical on op- 

 posite sides. 



g. The same at 15 — 18 days {oi. i. /). Transverse 

 across the middle of the trunk, for comparison 

 withy] 



a. The epiblast; well-defined and pigmented. 

 Examine under a high power, and note the 

 order of succession and characters of its layers. 



y8. The hypoblast and lower layer cells. Cf f. y. 



y. The neural tube (cerebro-spinal axis) pigmented 

 and medio-dorsal, enclosing a central canal ; it 

 no longer shows any trace of connection \vith 

 epiblast. (Cf. i. h) 



8. The notochord; immediately beneath y, large 

 and rounded, composed of large vacuolated cells. 



