502 PATTERNS AND PROBLEMS OF DEVELOPMENT 



it invaginates, but the data of neural induction do not support the view 

 that specific characteristics of the chorda-mesoderm are directly concerned 

 in determining that overlying ectoderm shall become neural tissue. They 

 may play a part in determining features of the pattern of its further de- 

 velopment and differentiation; but other tissues, living or dead, from 

 many organisms, tissue extracts, and residues and various synthetic sub- 

 stances can induce development of neural tissue from ectoderm. Again 

 the question arises: Is this induction anything more primarily than acti- 

 vation? If this induction is not specific, the chorda-mesoderm is only in- 

 directly neural inductor; it activates ectoderm, and the activated ecto- 

 derm becomes neural tissue. Later in development, when ectoderm has 

 attained a somewhat different condition, an activation may result in de- 

 velopment of a lens, at least in head ectoderm. The epidermis has very 

 probably undergone some degree of differentiation which makes lens de- 

 velopment possible with sufficient activation by an inductor or otherwise. 



PRESENT STATUS OF THE INDUCTOR PROBLEM 



By way of conclusion to this chapter the attempt is made to indicate 

 briefly the conception of induction to which the evidence, at present 

 available, points. It is impossible to draw a hard and fast line between 

 what is quantitative and what is specific in living protoplasms; but, ac- 

 cording to the evidence considered in preceding chapters, axiate organis- 

 mic pattern in its early stages appears to be predominantly or wholly a 

 quantitatively graded pattern, always, of course, in a protoplasm of spe- 

 cific constitution. Nonspecific alterations of that pattern alter the course 

 of development. Even a difference in oxygen tension may determine the 

 physiological and morphological differences between a stolon and a hy- 

 dranth in certain hydroids (pp. 172-75). 



In general, the natural inductors of early stages are, or at certain de- 

 velopmental stages become, high gradient-levels, and their inducing action 

 apparently consists in alteration of condition in regions representing 

 lower levels. This alteration evidently involves activation; and, with the 

 progress of experimental analysis, it appears increasingly probable that 

 from the hydroids to the vertebrates the primary factor in the inductions 

 of early development, whether reconstitutional or embryonic, is activa- 

 tion. Amphibian head-, trunk-, and tail-inductors, for example, represent 

 different gradient-levels of the inductor region ; and there is no conclusive 

 evidence that at the time of gastrulation they differ in other ways, though 

 they may already be different from the ectoderm. Moreover, their indue- 



