AXIAL SUSCEPTIBILITY GRADIENTS IN THE EARLY 

 DEVELOPMENT OF THE SEA URCHIN. 



C. M. CHILD. 



(WITH 20 FIGURES.) 



Axial gradients in susceptibility to cyanides and various other 

 agents have already been demonstrated in Planaria (Child, 136) 

 various infusoria (Child, '14), the early developmental stages of 

 the starfish (Child, '150), in a number of species of Oligo- 

 chetes (Hyman, '16), in a number of algae and in various 

 other animals the data for which are as yet unpublished. Thus 

 far such gradients have been found, at least in 'the earlier stages 

 of development in all forms examined, comprising more than 

 sixty species and including algae, ccelenterates, flatworms, echino- 

 derms, annelids, and vertebrates. The relation of these gradients 

 to developmental gradients of other kinds and the problem of 

 their significance for the physiological individual has also been 

 considered (Child, '156, Pt. Ill, 'i$c). 



During the summers of 1913 and 1915 at the Marine Biological 

 Laboratory, Woods Hole, Mass., axial susceptibility gradients 

 were demonstrated in the early developmental stages of the sea 

 urchin, Arbacia punctulata, and the control and modification of 

 development by means of the differential susceptibility along 

 these gradients was found to be possible. In the present paper 

 only the direct evidence for the existence of such gradients, 

 based on the progress of death and disintegration along the axis, 

 is considered and this is incomplete in certain respects. The 

 indirect evidence from the control and modification of develop- 

 ment, with which I was chiefly concerned and which is of greater 

 interest, will be presented in another paper. 



METHODS OF DEMONSTRATING SUSCEPTIBILITY GRADIENTS. 



The method employed in demonstrating the gradients has 

 already been described (Child, '130, '156, Chap. III.) and con- 

 sists in directly determining the differences in susceptibility along 



