306 



Embryogenesis: Progressive Differentiation 



movements are not rapid enough to account 

 for such results; the most striking potency 

 change occurs at the onset of gastrulation, 

 before convergence becomes really noticeable 

 (cf. Fig. 115). This is the latest stage when 

 an axis may be induced in the ventral lip 

 by transplantation of a piece of dorsal lip; 

 it is, also, the stage when defects in the 

 dorsal lip region begin to be incompletely 

 regulated (Devillers, '48b). 



In the avian egg, organization of the blas- 

 tula stage has been most successfully studied 

 by means of sectioning the blastoderm in 

 situ, thus isolating various portions mechani- 

 cally. In the duck egg, which is relatively 

 younger at the time of laying than the 

 chick, and may be classified as a blastula, 

 Lutz ('48) has accomplished the startling 

 feat of obtaining four embryos from one 

 blastoderm. This result could be obtained 

 by making two perpendicular or three paral- 

 lel cuts in the blastoderm. It would seem 

 that all peripheral areas of the pellucid 

 area (at least) were equally able to regulate 

 a whole when isolated; any fixed pre-locali- 

 zation of an inducing center during cleavage 

 would thus be excluded, for this form. It 

 must be recalled that this sectioning of the 

 avian area pellucida is not the morphologi- 

 cal equivalent of Luther's experiments on 

 the trout: the relation to the future blasto- 

 pore is quite different in the two (cf. Fig. 

 111). 



For the chick, the transplantation experi- 

 ments of Butler ('35) and the injury experi- 

 ments of Twiesselmann ('38) on early gastrula 

 stages indicate that at this time axial organ- 

 izer material becomes localized in the poste- 

 rior sector of the pellucid area. At the same 

 period, Waddington ('33) has demonstrated 

 that an axis may be induced in the anterior 

 epiblast, which normally would become ex- 

 traembryonic, by contact with the posterior 

 hypoblast. 



As the primitive streak elongates, it ap- 

 pears to contain the material directly capa- 

 ble of performing inductions of neural axis; 

 it must be noted that regions outside the 

 streak seem not to have been systematically 

 tested for this action, so that there is really 

 no experimental justification for stating that 

 inducing action is restricted, in the living 

 blastoderm, to the primitive streak. Wood- 

 side ('37) has shown that the capacity to re- 

 spond to induction — i.e., neural competence 

 — at first evenly distributed throughout the 

 whole disc, falls off with progressing gastrula- 

 tion, becoming by degrees restricted to the 

 area which normally forms medullary plate. 



Figure 115 is intended to summarize the 

 substance of the discussion in the preceding 

 paragraphs. It has been emphasized that it 

 has not been possible to apply strictly com- 

 parable tests in all cases. The general trend, 

 however, is alike in all forms: namely, that 

 both induction potency and neural com- 

 petence are at the start of gastrulation widely 

 distributed; that the induction center early 

 becomes restricted to the invaginating area; 

 and that considerably later neural compe- 

 tence likewise becomes restricted to the 

 medvillary plate region. By the neurula stage, 

 responsiveness to the axis-inducing stimulus 

 evidently is at an end; the ability to induce 

 lingers on in axial tissues and may be un- 

 masked in many adult tissues by coagulation. 



PRIMITIVE ORGANIZATION OF THE 

 EMBRYO 



THE ECTODERM 



Events of the gastrulation period are most 

 strikingly viewed if attention is focused, as 

 has been done in the preceding section, on 

 the primary embryonic areas (the invaginat- 

 ing and the non-invaginating regions), and 

 on the primary induction of the dorsal axis. 

 That this is an oversimplification is obvious. 

 Each of these primitive areas is to play a 

 role as germ layer first, segregating sub- 

 sequently along anteroposterior and medio- 

 lateral axes into definite regions which are 

 traceable directly to the organs of the adult 

 body. Differentials in the future germ layers 

 may be recognizable before, and especially 

 during, the invagination process. 



In the late gastrula and neurula stages, 

 transplantation experiments in both teleost 

 and chick agree in showing a more or less 

 fixed anteroposterior arrangement of medul- 

 lary ectoderm. Figure 116 has been pre- 

 pared to summarize the results of numerous 

 investigations of this nature; the axiation 

 is clearly indicated in all cases that have 

 been reported. In many of these cases, how- 

 ever, the transplanted piece contained both 

 archenteron roof and overlying ectoderm; 

 the subsequent differentiation into appro- 

 priate levels of the central nervous system 

 might thus be attributed to specificity of 

 either layer, or of both. 



The anterior tip of the neural axis, in 

 particular, presents special problems. It 

 would be expected that the anteriormost 

 invaginated material (prechordal plate) 

 would be the normal inductor of forebrain 

 tip and optic vesicles. The elegant experi- 

 ment of Luther ('36b) on Salmo, in which 



