452 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



fer fundamentally in general pattern of embryo formation from that of 

 other vertebrates. Moreover, pattern of embryo formation in Amphioxus 

 and ascidians shows certain general resemblances to that of vertebrates. 



B 



Fig. 153, A-D. — Regional map of chick embryo at four stages; dotted area, lateroventral 

 mesoblast; oblique or vertical lines, somite region; coarse stippling, cephalic mesoblast; hori- 

 zontal lines, notochord; neural region indicated by line marking its anterolateral boundary, 

 .1, early blastoderm; B, early primitive streak, parallel dotted lines indicating streak; C J). 

 later stages, showing superficial map on left side, map of invaginated parts on right (after 

 Pasteels, 19366). 



As already pointed out, the regional patterns indicated by the maps 

 give no information concerning determination or differentiation of the 

 regions distinguished. At best they are maps of future embryonic parts. 



