APPENDIX IV 



The general coincidence of susceptibility and dye-reduction gradients and the 

 similarity of these gradients observed in related species raises certain questions con- 

 cerning the apparent absence of a secondary acropetal susceptibility gradient in the 

 basal region of the sea urchin Arbacia and the starfish Asterias. A re-examination of 

 these forms as regards both susceptibility and dye reduction is desirable; but, lacking 

 this, certain possibilities may be noted. The change in condition in the basal region 

 may conceivably occur later in Arbacia and Asterias than in the othei species studied, 

 though this appears rather improbable in view of the general similarity in all, except 

 as regards absence of mesenchyme formation preceding gastrulation in asteroids. 

 The laboratory notes on which the earlier papers on susceptibility of Arbacia and 

 Asterias (Child, i9i5«, 1916a) were based give some evidence of increased suscepti- 

 bility about the blastopore in both forms after gastrulation has begun. At the time 

 the papers were written, this was thought to be associated, at least in Asterias, with 

 contraction of the blastopore observed in this form. In the observations on differ- 

 ential death in Arbacia it was noted that in late blastulae cells were sometimes given 

 off from the midbasal region, the region of primary mesenchyme; but the possible 

 significance of this fact was not realized. Some of the differential modifications of 

 Arbacia development by external agents give evidence that susceptibility of the ento- 

 derm does increase at some stage of development; but, since they result from exposure 

 to the agent during the whole, or a considerable, period of development, it is not pos- 

 sible to determine from them at what stage the change occurs. Irregularities in differ- 

 ential death, that is, cytolysis of a cell or a few cells earlier than others at that body- 

 level, are not infrequent and have been thought perhaps to indicate that these cells 

 were at a more susceptible stage of the division cycle than those about them. The 

 cytolysis of basal cells in Arbacia was assumed to be due to this or some other incidental 

 factor. As regards the differential modifications of development, attention was direct- 

 ed chiefly to the modifications of external form and proportions; and while the condi- 

 tion and degree of entodermal development were recorded and drawn, the question of 

 relative inhibition of entoderm and ectoderm received little consideration. Re-exami- 

 nation of these notes on the early studies of susceptibility indicates that attention was 

 so sharply focused on the primary basipetal gradient and the developmental modifi- 

 cations resulting from it that the significance of evidences of a secondary acropetal 

 gradient was not realized. They seem to be an excellent example of failure to grasp the 

 meaning of what is actually seen and recorded because attention is directed elsewhere. 

 It appears possible, however, that the change in condition in the basal region of 

 Arbacia is less extreme than in Strongylocentrotus. The invaginated entoderm of 

 Arbacia is larger in relation to size of the blastocoel, and its wall is thicker than in 

 Strongylocentrotus; also, it does not undergo so much elongation in reaching the stomo- 

 deal region, and its cells do not change in shape so greatly during these stages as in the 

 latter form. These differences may signify that the change in condition in the basal 



739 



