446 PATTERNS AND PROBLEMS OF DEVELOPMENT 



embryo, is apparently not necessary for invagination. Stained cells from 

 vegi, implanted in the apical poles of whole embryos, may invaginate with- 

 out reconstituting primary mesenchyme; but a part of the primary mesen- 

 chyme of the host aggregates about the invaginated cells (Horstadius, 1935) . 

 In view of all the evidence, not only that from the transplantation ex- 

 periments but also that from differential dye reduction, differential sus- 

 ceptibility, and the differential modifications of development resulting 

 from it, it may still be questioned whether induction by implanted micro- 

 meres is primarily anything more than a nonspecific effect depending on 

 physiological condition rather than on specific differentiation; but even 

 if the micromeres produce a specific substance, its effect may be primarily 

 activation. In his interpretations of experimental results Horstadius uses 

 terms implying dynamic factors. He speaks of the "conflict" of the gra- 

 dients, the "strength" or "weakness" of one or the other, the "suppres- 

 sion" of one by the other, the "weakening" of one, permitting the other 

 to become stronger and gain the upper hand. In terms of primarily non- 

 specific gradients of rate or intensity sea-urchin development appears not 

 as a conflict of gradients but rather as an orderly and definitely deter- 

 mined sequence of gradient changes which can be altered in definite ways 

 by changes in relations of parts and in environment. 



FORMAL PATTERNS OF VERTEBRATE DEVELOPMENT 



Before discussing induction in vertebrate development some considera- 

 tion of the formal regional maps and of the cell movements or migrations 

 by which parts of the early embryo attain their final positions is neces- 

 sary. In the study of embryonic development it soon became evident, 

 for vertebrates as for invertebrates, that observation and description of 

 development, necessary though it was as a first fine of attack, could give 

 us httle more than a timetable of events as a basis for discussion and 

 speculation. The observation that localization of the embryo in a par- 

 ticular region involved considerable changes in position and apparent mi- 

 gration of cell materials raised the question of the fate in development, 

 the prospective significance, of different regions; and the theory that em- 

 bryo formation resulted in large part from concresence of two lateral 

 halves was advanced and debated pro and con. In the attempt to de- 

 termine more exactly the fates of different regions methods of local mark- 

 ing by puncture or other local mechanical injury, insertion of hairs, local 

 cautery, and, later, local electrolytic injury and radiation were employed. 

 To all these the objection has been raised that the injury or obstruction 

 resulting from it may interfere with the cell migrations. The method of 



