Teleosts and Birds 



ectoderm that would normally have re- 

 mained extraembryonic was placed in the 

 path of otherwise undisturbed invaginating 

 archenteron roof, resulted in induction of 

 forebrains that were normal in many cases. 

 Some abnormal indvictions resulted, and 

 some undersized forebrains as well. For Fun- 

 dulus, as Figure 116 A shows, forebrain and 

 eye differentiate from transplants of anterior 

 embryonic shield pieces even if mesoderm 

 is not included, although better if the under- 

 lying material is present. It should be men- 

 tioned that in Fundulus the anterior end 

 of the notochord lies at the hindbrain level, 

 so that the prechordal inductor must be 

 relatively extensive. The above-mentioned 

 grafts, however, developed in the host em- 

 bryonic shield, and were thus exposed to 

 undefined host induction fields. In the ex- 

 periments cited in the preceding section, 

 where an axis was induced by transplanted 

 dorsal lip, forebrain defects were reported 

 to be of almost imiversal occurrence in the 

 induced axis. Under the hypothesis that the 

 anteriormost cells invaginated are capable 

 of inducing forebrain, specifically, these de- 

 fects would be attributed to the absence of 

 a definite region of the transplanted in- 

 ductor. Less compatible with this hypothesis 

 is the failure (Fig. 11 6B) of anterior archen- 

 teron roof to indvice eye or forebrain under 

 conditions where posterior levels of the same 

 structure perform appropriate inductions. 

 It will be recalled that a highly effective 

 method of producing cyclopia in Fundulus 

 involves exposure to deleterious agents in 

 early cleavage stages (Stockard, '07). These 

 considerations make it necessary to believe, 

 at the very least, that if the localized in- 

 ductor hypothesis is true, there must also 

 be special additional factors in the anterior 

 tip of the axis, much more easily disturbed 

 by environmental changes than are the rela- 

 tions in the rest of the dorsal axis. 



In the chick, peculiarities of the anterior 

 end of the neural axis appear in the period 

 of the formation of the primitive streak (Fig. 

 116Z>). At this time, isolated anterior pieces 

 of epiblast form unspecific medullary plate 

 or tube when grown in vitro, even before 

 mesoderm has begun to underlie the upper 

 layer in that region. In such preparations, 

 mesenchyme is invariably present; one pos- 

 sible interpretation is that a sporadic in- 

 vagination from the upper layer may simu- 

 late mesoderm formation together with its 

 induction effect. In the late gastrula stages 

 (Fig. 116£), as the anterior ectoderm be- 

 comes fully underlaid, forebrain and optic 



307 



vesicles tend to form recognizably in vitro 

 even when isolated from posterior levels; this 

 tendency becomes progressively fixed as the 

 medullary plate becomes visible. During this 

 period, potency to form retinal tissues in 

 grafts is evenly distributed along the medio- 



/ r~J-\- FOREBRAfN 



I 02 1- MIDBRAIN 



/ □ V HINDBRAIN 





FOREBRAIN 

 EYE 



HINDBRAIN 

 CORD 



Fig. 116. Axial differentiation of central nervous 

 system. A, Results of transplants or excisions from 

 the axial keel of Fundulus (after Oppenheimer, 

 '36b, Fig. 21). B, Results of inductions by three 

 levels of the archenteron roof of Salmo, associated 

 with extraembryonic ectoderm (after Eakin, '39, 

 Fig. 1 ) . C, Results of transplantation of parts of the 

 unincubated chick blastoderm to the chorioallantois 

 (Butler, '35). D, Explantation of pieces of the chick 

 blastoderm: pre-groove primitive streak stage 

 (Spratt, '42). E, Localization of central nervous 

 system levels in the definitive primitive streak stage 

 of the chick (data from Hunt, '32; Clarke, '36); 

 chorioallantoic transplantation. F, Same for the 

 head-process stage (from Clarke, '36; Rawles, '36). 



lateral axis of the eye-forming area; the 

 localization of histogenetic potency lags be- 

 hind morphogenetic localization. Ultimately 

 the ability to differentiate retina is localized 

 in the optic vesicle areas, shortly before they 

 themselves become morphologically patent. 

 In the definitive primitive streak and 

 head-process stages, transplantation experi- 

 ments reveal anteroposterior axiation of the 

 future medullary plate (Fig. 116E,F), with 



