222 



BIOLOGICAL PHYSICS 



axonal cell processes and the inter-penetration and blend- 

 ing of their structural elements, with ultimate intimate 

 union of their respective terminal nervine extensions, 

 and, consequent complete in nervation, unal and dual, 



FIG. 82. TRANSVERSE SECTION OF AN EMBRYO CHICK IN THE LATTER 

 HALF OF THE SECOND DAY, AT THE PLACE WHERE THE VERTEBRAL 

 SOMITES CEASE. - S T 3 - (From Kolliker.) 



rw, dorsal ridges : rf, medullary groove or canal beginning to close ; itwp, proto- 

 vertebral plate ; sp, lateral plate of the mesoblast ; h, epiblast ; del, hypoblast ; ao, 

 primitive right aoria ; sp, commencement of division of the mesoblast which 

 forms the body cavity. 



single and combined. At this stage of the organic pro- 

 gress of evolution of the human organism, the most 

 notable feature is the neurenteric canal (Figs. 84, 85), 

 which is most happily named, as it is a canal in every sense 



uwh 



FlG. 83. TRANSVERSE SECTION THROUGH THE KMBRYO OF THE CHICK 

 AND BLASTODERM ON THE SECOND DAY. (From Kolliker.) 



t/J, hypoblast ; ch, chorda dorsalis ; tew, primordial vertebrae ; tnr, medullary plates ; 

 /z, corneous layer or epiblast ; iiivh, cavity of the primordial vertebral mass ; mp, 

 mesoblast dividing at sp into kpl, parietal, and df, visceral laminae ; ung, Wolffian 

 duct beginning in the intermediate cell-mass. 



of the word, and contains a common fluid, stretching from 

 the buccal cavity to the anterior vesicle, or rudiment of 

 the future brain, with an unbroken lumen and continuous 

 walls. It is V shaped, becoming open at its buccal end, 

 and closed at its cerebral vesicular termination, its two 

 halves becoming differentiated and partially separated, at 

 the posterior extremity of the V, and united by anasto- 



