DEVELOPMENTAL EVOLUTION 



233 



servient to the materio-dynamic necessities of the two 

 great nervatures. 



In virtue of these latter additions to the mechanisms, 

 and agencies of embryonic life, further developmental 

 progress follows, with constantly increasing degrees of 

 advancement in structural complexity and functional 

 attainment, comprising the rudiments of the brain, cord, 

 and nerves, and a bond of union between these, and the 

 presiding sympathetic nervous system, which, until now, 



FIG. 94. 



FIG. 95. 



FIG. 94. PYKIFORM TRANSPARENT AREA OF THK CHICK'S BLASTODERM 



WITH THE PRIMITIVE GROOVE. 



pr, primitive streak and groove ; af ; amniotic fold commencing ; the darker shading 

 round the primitive streak indicates the extension of the mesoblast. 



FIG. 95. EMBRYONIC AREA FROM THE OVUM OF A RABBIT OF EIGHT 

 DAYS. - 2 r 2 - (From Kolliker.) 



arg, border of the embryonic area ; pr, primitive streak with groove. 



has met all the requirements of a general nervous system 

 and determining agent, in the work of developmental 

 evolution. 



Now comes a momentous period in the history of 

 embryonic advancement, when the foundation, of the 

 future systemic nervous system, is laid in the posterior 

 and half of the neurenteric canal, and when the division of 

 the canal into two takes place, and its anterior division is 

 converted into the alimentary and respiratory systems, 

 of viscera and organs, and, inasmuch, as this great process 

 of structural and functional, differentiation is accompanied, 



