J 68 ELEMENTARY BIOLOGY. [CIIAP. 



a. 'I'he neural plate; a thickened mcdio-dorsal 

 sheet of epibhast, partly invaginated and en- 

 closing a deep neural groove; its raised edges 

 {neural folds). 



f]. 'I'hc arehenteron ; 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 yG: its cells, somewhat 

 flattened above, graduating into the lower layer 

 cells laterally. 



8. The notoe/ionl ; a median o\'al mass of small 

 cells, immediately beneath the neural plate, 

 not yet completely constricted off from the 

 hypoblast. 



e. The mesohlast ; interposed between the epiblast, 

 and hypoblast with the lower layer cells; mo.^t 

 niarked dorso-laterally and symmetrical on op- 

 l)Osite 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 i)igmented. 

 ICxamine under a high power, and note the 

 order of succession and characters of its layers. 



/j. The hypoblast and lo7c>er layer eel Is. Ci. /. y. 



y. The neural tube (cerebrospinal axis) i)igmented 

 and medio-dorsal, enclosing a eentral eanal; it 

 no lon2:er shows anv trace of connection with 

 epiblast. (Cf. i. //.) 



8. 'J'he notoehord; immediately beneath y, large 

 and rounded, composed of large vacuolated cells. 



