Teleosts and Birds 



303 



In both types, it should be noted, con- 

 vergence and concentration of material that 

 is to enter the embryonic axis is in sharp 

 contrast to thinning and rapid epibolic 

 spreading of material destined to be extra- 

 embryonic. Luther ('37) was able to show 

 an antagonism between embryo formation 



EARLY GASTRUCA; 

 (UNINCUBATED)' 



HYPOBLAST DELAMINATED 

 FROM POSTERIOR HALF 



tion from the rest of the blastoderm. Future 

 ventral or extraembryonic areas react to ex- 

 plantation in vitro by spreading and vesicu- 

 lating (Rudnick, '38b). 



In embryonic and extraembryonic areas, in 

 teleost and chick, before and during gastru- 

 lation, the innate migration tendencies of 



EPIBLAST. 



EARLY PRIMITIVE STREAK 



ECTODERM 



ANTEROt"' 

 POSTERIOR 



LATE PRIMITIVE STREAK 



HEAD -PROCESS 



Fig. 113. Gastrulation stages in the chick: prospective areas, from marking experiments by "Wetzel ('29), 

 Pasteels ('36b), Spratt ('46, '47, '52), Spratt and Condon ('47). In all but the miincubated stage, the left half 

 shows superficial areas, the right half, invaginated mesoderm. The following prospective areas are indicated: 

 extraembryonic ectoderm, shaded. Embryonic ectoderm, white: the medullary plate is enclosed by a heavy 

 line and its anteroposterior levels demarked in the head-process diagram as P, prosencephalon; M, mesen- 

 cephalon; R, rhombencephalon; Sp, spinal cord; T, tail nerve cord. Chorda and prechordal plate, stippled. 

 Mesodermal somites, heavy parallel lines. Heart, cross-hatched. Lateral plate and extraembryonic mesoderm, 

 light horizontal lines. The blastopore or primitive streak is indicated by vertical shading. The boundaries of 

 the posterior and lateral mesodermal areas are conjectural; those of the anterior axis (notochord and somites) 

 as well as of the ectodermal areas have been carefully mapped by Spratt and his co-workers, on explanted 

 blastoderms. In the head-process stage, the region of greatest anteroposterior growth, most pronounced in the 

 median line, is bracketed. (Spratt, '47; Gaertner, '49.) 



and epiboly by defect experiments in Salmo. 

 In this same form, Devillers ('48a) has shown 

 that the onset of gastrulation introduces a 

 marked localization of the spreading tend- 

 ency, which becomes restricted to the fu- 

 ture extraembryonic region at that time 

 (cf. Fig. 114). This may indicate the first 

 appearance of a relatively persistent physio- 

 logical differentiation between these two 

 major areas. 



In the chick, explantation experiments 

 show a similar characteristic tendency of 

 axial materials to condense, even in isola- 



the epithelia do not interfere with wound 

 healing, or with regulation of excised parts, 

 provided the latter are not disproportion- 

 ately large. The role of the surface coat in 

 this process has been studied for the trout 

 egg by Devillers ('48a, '51b), in Fundulus 

 by Trinkaus ('49, '51). No surface coat as 

 such has been demonstrated in the chick 

 blastoderm or yolk, but the behavior of the 

 former after lesions would indicate that a 

 continuous intercellular material must be 

 present and must function similarly after 

 wound healing. 



