EMBRYONIC INDUCTORS AND ORGANIZERS 



451 



in relation to gastrulation, in Figure 152, B. Maps by Oppenheimer 

 {igs6d) are very similar. Regional maps of the chick blastoderm by 

 Graper (1929), Wetzel (1929), Waddington (1932), Pasteels (19366), and 

 Jacobson (1938) are, in general, much ahke but differ in certain minor 

 points which cannot be considered here. Maps of four stages by Pasteels 

 are shown in Figure 153, A-D; and in Figure 154, A-D, directions of cell 

 migrations are indicated. Superficial migrations in formation of the primi- 



A 



Fig. 152, A, B.—A, map of prospective embryonic regions of teleost blastoderm just pre- 

 ceding gastrulation; B, directions of cell migrations in gastrulation. Vertical lines, prechordal 

 plate; lightly stippled, lateral and ventral mesoderm; heavily stippled, caudal unanalyzed 

 regions; b, brain; t, trunk ectoderm; c, notochord; ce, cephalic ectoderm; e, extraembryonic 

 ectoderm; n, postcephalic nervous system; s, somites (from Pasteels, 1936a). 



tive streak, as determined by Wetzel, are shown in Figure 155, A-C. 

 Jacobson beheves that the longitudinal migration recorded by these au- 

 thors does not take place. In spite of the general similarity of results. 

 there has been disagreement as regards significance of the primitive streak. 

 The more generally accepted view is that the streak represents a part 

 of the gastrulation process in which mesoderm is invaginated, entoderm 

 being invaginated earlier from the posterior border of the blastoderm 

 (e.g., Patterson, 1909). Wetzel, however, maintains that the streak has 

 nothing to do with gastrulation. 



It has become evident that the formal regional pattern and the migra- 

 tions concerned in embryo formation are, in general, similar in all verte- 

 brate groups. Even mammalian development, although it exhibits certain 

 features associated with intrauterine environment, evidently does not dif- 



