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 archenteron; 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 ft : its cells, somewhat 

 flattened above, graduating into the lower layer 

 cells laterally. 



8. 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 (cf. i. /). Transverse 

 across the middle of the trunk, for comparison 

 with/ 



a. The epiblast; well-defined and pigmented. 

 Examine under a high power, and note the 

 order of succession and characters of its layers. 



ft. The hypoblast and lower layer cells. Cf./. y. 



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

 and medio-dorsal, enclosing a central canal ; it 

 no longer shows any trace of connection with 

 epiblast. (Cf. i. /.) 



3. The notochord; immediately beneath y, large 

 and rounded, composed of large vacuolated cells. 



