GERM DISC \ /, '■'-'•'•'.•l o 



PRIMITIVE STREAK ' iNj'-l B. 



AREA onikjiTi\/i 



NOTOCHORO 



ENSEN'S NODE 



PRE<HORDAL PLATE 



NOT OC HORD 



HE N sen's nod 



PRIMITIVE MESO 



NECK 



PRIMITIVE STR 



MAR ei N OF E MB 

 DISC 



Fig. 208. Development of primitive streak, notocliord, and mesodermal migration in 

 the pig. (After Streeter, '27.) (A) Primitive streak represented as thickened area at 

 caudal end of embryonic (germ) disc. Migrating mesoderm shown in heavy stipple. 

 (B-E) Later stages of streak development. Observe mass migration of mesoderm. The 

 mesoderm outside the germ disc is extra-embryonic mesoderm. (F) Forward growing, 

 primitive streak makes contact with Hensen's node. (G-1) Observe elongation of 

 notochord accompanied by recession of primitive streak shown in (I). Observe in (I) 

 that an embryo with three pairs of somites has formed anterior to Hensen's node. Com- 

 pare with Spratt's observation on developing chick, fig. 206, type VI. 



434 



