204 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the case, it appears that the same inhibiting agent may entodermize pro- 

 spective ectoderm in early stages, when it represents higher gradient-levels 

 than entoderm, and ectodermize prospective entoderm in later stages, 

 when it has attained a higher gradient-level than ectoderm. Further 

 examples of this differential inhibition will be given in the section on 

 exogastrulation. 



The so-called "sterroblastula," appearing so frequently under widely 

 varied inhibiting conditions and often in pathological lots of eggs without 

 external inhibition, is a blastula in form with blastocoel more or less com- 

 pletely filled with cells. It represents a differential inhibition with dis- 

 sociation of cells from the entodermal region and perhaps also from other 

 parts of the wall. Apparently, all that is necessary to produce sterroblas- 

 tulae is a sufficient degree of inhibition. In general, the developmental 

 modifications resulting from differential inhibition differ with concentra- 

 tion or intensity of agent, with exposure period, and with stage when ex- 

 posure begins; but there is no conclusive evidence of specific regional 

 effects of particular agents: the differences appear rather to be indicative 

 of quantitative factors in physiological condition and their changes in the 

 course of development than of specific factors. 



SECONDARY MODIFICATIONS: DIFFERENTIAL TOLERANCE, 

 CONDITIONING, AND RECOVERY 



These secondary modifications of development follow an initial inhibi- 

 tion; they are much greater and occur much more rapidly with some 

 agents than with others. With cyanide, for example, they occur slowly 

 and are not extreme; with ethyl alcohol, acidified sea water, hypotonic 

 sea water, and various other agents they appear earlier and may be ex- 

 treme; with CUSO4 and MgCl^ they appear somewhat more slowly but 

 may still be great. ^ 



With slight inhibition in early stages the first evidence of differential 

 tolerance or conditioning is relative elongation and enlargement of the 

 apical region; this may appear in the gastrula with slight inhibition by 

 agents such as ethyl alcohol, acidified or hypotonic sea water, and various 

 others (Fig. 77, A,B), but with many agents it becomes evident somewhat 

 later. In the prepluteus the oral lobe becomes relatively large and long 

 (Fig. 77, C), and the resulting pluteus has a large oral lobe and a wide 

 brachial angle up to 180°, a relatively large ventral, and a small dorsal 



" Arhacia has been the chief material in these experiments, but data on Strougylocentrotiis 

 and Dendraster are sufticient to show that the secondary modifications in these species are 

 essentially similar. 



