THE AXIAL GRADIENTS IX HYDROZOA. 



VI I. MODIFICATION OF DEVELOPMENT THROUGH DIFFERENTIAL 



SUSCEPTIBILITY. 



c. M. CHILD. 



From the Hull Zoological Laboratory, the L'niversity of Chicago. 



In the preceding paper of this series the physiological analysis 

 of embryonic development in certain species of hydrozoa was 

 begun with a study of the physiological gradients and their 

 changes during the usual course of development (Child, '25). 

 The present paper is a continuation of this analysis and is 

 concerned with the modification of development resulting from 

 the differential susceptibility to various agents of different levels 

 of the polar gradient or gradients. 



MATERIAL AND METHOD. 



Developmental stages of the leptomedusa, Phialidium gre- 

 garium, abundant in Puget Sound, constituted the chief experi- 

 mental material. Since the medusae shed eggs or sperm more or 

 less continuously early developmental stages are readily obtained 

 by keeping the sexually mature medusae in large containers for a 

 time and then collecting the embryonic stages from the bottom. 

 Developmental material obtained in this way shows a certain 

 range of stages, depending on the length of time the medusae are 

 kept in the container, but by using large numbers of medusae 

 sufficient material for one or more experimental series can usually 

 be obtained within a few hours. With this procedure the stages 

 range from newly fertilized eggs to more or less advanced cleavage 

 or early blastulae and still greater uniformity can of course be 

 obtained by sorting the stages, but this is unnecessary for most 

 purposes. A few experiments were performed with the early 

 planulae of Gonothyrcca darkii after their emergence from the 

 gonophores. 



The experimental procedure consisted in the exposure of the 

 developmental stages to concentrations of agents which had been 



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