ON THE STAGES OF EVOLUTION 221 



by which we are enabled to trace the sequence of events 

 in the past, and dimly to perceive the probable trend of 

 events in the future. 



FIG. 80. DORSAL VIEW OF A BLASTODERM AND EMBRYO CHICK HAVING 

 FIVE MESOBLASTIC SOMITES. (From Balfour. ) 



a.pr, anterior part of the primitive streak ; p.pr, posterior part ; the medullary ridges 

 have come together in the greater part of their extent, but have not yet united ; 

 the caudal swellings are visible on each side of a.pr. 



The systemic nervous system is evolved, from the 

 ectodermal division of the sympathetic (Fig. 79), nerv ne 

 area, by the growth and involution, of the dorsal segment 



FIG. 81. -TRANSVERSE SECTION THROUGH THE EMBRYO OF THE CHICK 



AND BLASTODERM AT THE END OF THE FIRST DAY. Magnified from 



90 to ioo times. (From Kolliker.) 



A, epiblast ; dd, hypoblast ; sp, mesoblast ; Pz>, medullary groove ; in, medullary 

 plates ; ch, chorda dorsalis ; uwf>, proto-vertebral plate ; uw h, commencement of 

 division of mesoblast into its upper and lower laminae ; between /f/'and h are the 

 dorsal laminae or ridges which by their approximation close in the medullary 

 canal. 



of the blastoderm (Fig. 80), and to some extent invades, 

 and innervates its hypodermal, or ventral, segment, from 

 which it becomes ultimately differentiated, but not ab- 

 solutely separated, thereafter ; its further evolution, and 

 distribution to the mesodermal (Figs. 81, 82, 83) and 

 hypodermal areas, being secured by elongation of its 



