Teleosts and Birds 



309 



culiarities of the ectoderm at the tips of 

 the limb buds in the chick, point to how 

 much there remains to learn about the prop- 

 erties of non-neural ectoderm. 



the chorda field in the chick gastrula (Fig. 

 117 E,F) is in striking contrast, and would 

 indicate that this structure, at least, is local- 

 ized relatively early in the latter form — a 



THE LOWER LAYERS 



As gastrulation progresses, not only do in- 

 duction capacity, neural competence, and 

 the specific competences for different levels 

 in the ectoderm become gradually restricted 

 to the appropriate embryonic area: the 

 invaginating layers themselves ultimately 

 must become a mosaic of areas of definite 

 differentiation tendencies. It is believed that 

 all these areas have at first the properties 

 of fields, of indefinite boundaries, possibly 

 overlapping one another. Toward the time 

 of beginning morphogenesis, the boundaries 

 of the fields evidently become more restricted 

 and more precise, as if brought into focus 

 by some microscope capable of reading the 

 future; each field then coincides with the 

 corresponding organ-forming area. 



In the forms vmder discussion, our experi- 

 mental knowledge of this course of events in 

 the lower layers is based only on the results 

 of transplantation experiments in which at 

 least two layers were involved, and which 

 resulted in differentiation of complexes of 

 several tissues. The desirable isolation ex- 

 periment has not yet been technically pos- 

 sible. The succession of figures which form 

 the matter of the present section is an ab- 

 straction, presented in order to help the 

 reader to visualize, for each major embryonic 

 structure, the spatial distribution of its 

 origin in grafts, delimited as well as the 

 experimental material allows. Many of the 

 studies on which these abstractions are based 

 have not been carried through to the stage 

 where the potency field corresponds with 

 the actual organ-forming area; but the 

 tendency in that direction is clearly observ- 

 able. 



Figure 117 deals with the axial mesoderm, 

 in so far as notochord and somites (or skele- 

 tal muscle) are found to occur differentially 

 in grafts of parts of blastoderms. The re- 

 ported data are unsatisfactory for the blas- 

 tula stage; in particular the distributions 

 indicated for the chick (Fig. 117D,J) in 

 the unincubated stage are based on a few 

 positive cases and may not be valid. The 

 trout, however, shows an vmderstandable 

 sequence, with a notochord field converging 

 toward the dorsal lip (Fig. 117B,C), fol- 

 lowed by later invaginated somite material 

 (Fig. 117//,/). The strict delimitation of 



Fig. 117. Progressive changes in fields for pro- 

 duction of chorda and somites in grafts from Salmo 

 (Luther, '36a) and the chick (Butler, '35; Hunt, 

 '31, '32; Rudnick, '32; Rawles, '36). In the diagrams 

 for SaLmo, the left half of the figure shows the 

 actual percentages of differentiation in various sec- 

 tors, on which the scheme of the right half is based. 

 No such quantitative estimate has been attempted 

 in the case of the chick. Localization in blastula 

 stages is based on very incomplete information, 

 which may be of no significance. The marked re- 

 striction of chorda-forming material in the chick 

 (E,F) is based on reliable data. The extent of the 

 somite field in the chick {J,K,L) is based merely on 

 differentiation of skeletal muscle, and so is not 

 critical, nor are the lateral boundaries established. 



condition not reached even in the late gas- 

 trula in Salmo. 



The interpretation of this localization in 

 the chick is not easy, as examination of Fig- 

 ure 118 will lead us to reflect. Here we 

 have similar figures showing progressive lo- 

 calization of two more lateral mesodermal 



