302 



Embryogenesis: Progressive Differentiation 



the marked precocity of axis formation in 

 the large-yolked Salmo is thus made clear. 

 On the enlarged diagrams of the early gas- 

 trula at the top, prospective areas have been 

 projected in lateral view. As in the case 

 of all vertebrates so far studied, invagina- 



Fig. 112. Comparative lateral diagrams of pro- 

 spective areas in the early gastrula and diagrams of 

 gastrulation stages. White: prospective nervous 

 system. Stippled: prospective chorda. Oblique lines: 

 future somites. Shaded: extraembryonic area (yolk 

 sac). TB, tailbud material; Ent, entoderm. The 

 boundary between chorda and entoderm in the 

 Fundulus diagram is purely conventional; experi- 

 mentally it has not been specified. The asterisk in 

 the gastrulation series represents the migration of a 

 group of cells on the germ ring into the somite 

 mesoderm. (From Pasteels, '36a; Oppenheimer, 

 '36b, '37.) 



tion of material to lower layers is accom- 

 panied by marked convergence of both in- 

 vaginating and superficial material to the 

 dorsal midline of the future embryo. Hence, 

 prospective areas for nervous system, chorda 

 and other mesoderm extend at first some dis- 

 tance laterally to the position they will 

 later occupy. The convergence movements 



in the mesoderm are summarily indicated 

 by migration of the asterisk in the lower 

 figures. Contrary to what might be expected 

 a priori, the trout blastodisc, which has 

 so much centrifugal growth and stretching 

 to accomplish, likewise undergoes much 

 more marked convergence movements. Meso- 

 dermal trunk material in Fundulus lies 

 originally closer to the embryonic shield 

 than it does in Salmo. In both, ultimately, 

 the ventral lip enters the tailbud. 



For the avian embryo, we have almost no 

 comparative material for this period; our 

 information derives almost exclusively from 

 the egg of the domestic fowl. Surface dia- 

 grams are presented in Figure 113 for the 

 stages corresponding to those just indicated 

 for teleosts. What emerges from the some- 

 times conflicting studies is that the following 

 order of gastrulation takes place: (1) The 

 hypoblast is formed by a process of sporadic 

 delamination from the posterior region of 

 the pellucid area; this process is not shown 

 in the diagrams of Figure 113. (2) In the 

 same general region, the primitive streak 

 appears soon afterward as an irregular thick- 

 ening, followed by invagination and emigra- 

 tion of mesoderm beneath the superficial 

 layer. The first invaginated mesoderm moves 

 laterally and posteriorly, becoming extra- 

 embryonic. The posterior part of the streak 

 through which it invaginates thus corre- 

 sponds roughly to the ventrolateral blasto- 

 pore lips. (3) Gradually, more anterior ma- 

 terial is incorporated in the streak and 

 anterolateral emigration of mesoderm en- 

 sues; the front end of the streak thus corre- 

 sponds to the dorsal half of the blastopore. 

 The anteriormost axial mesoderm remains 

 in the lower layers of the anterior streak 

 for considerable time: this concentration of 

 material constitutes Hensen's node. Lateral 

 and extraembryonic mesoderm continues to 

 invaginate through the posterior three- 

 fourths of the streak even after the axial 

 mesoderm, as notochord and somites, is 

 elongating and differentiating anterior to the 

 node. 



This order of events in the chick is clearly 

 associated with convergence of superficial 

 areas toward the invaginating zone, the 

 most extensive migration being performed 

 by the more peripheral area pellucida re- 

 gions which enter the early (posterior) prim- 

 itive streak. The arrows in Figure \\\B-1 

 are intended to show the approximate extent 

 of convergence. The movements thus differ 

 in order, tempo and magnitude from those 

 outlined for the teleost. 



