AN EXPERIMENTAL STUDY AND A PHYSIOLOGICAL 



INTERPRETATION OF EXOGASTRULATION 



AND RELATED MODIFICATIONS IN 



ECHINODERM EMBRYOS. 



* 



JOHX \Y. MACARTHUR. 

 DEPARTMENT OF BIOLOGY, UNIVERSITY OF TORONTO. 



INTRODUCTORY. 



In a study of the means of altering and controlling the course 

 of deYelopment in echinids by electrolytes the writer had oc- 

 casion to note the action of lithium chloride in effecting remarka- 

 ble form changes in the early ontogeny of sand-dollars and sea- 

 urchins. An attempt was then made to analyze further these 

 effects and the similar ones so extensively studied by Herbst 

 and others, and to provide for the wealth of facts obtained some 

 more consistent and satisfactory explanation than has been 

 offered. 



Application to these cases of the susceptibility and differential 

 inhibition methods of Child has supplied what seems a secure 

 and reasonable basis for interpreting such puzzling teratological 

 forms as exogastrulse, "holentoblastula?," etc. For I believe that 

 the first impression of an exquisitely specific action of the Li' ion 

 is not sustained by a closer examination of the facts, and that 

 "lithium larvae" fall logically into the series of abnormal forms 

 producible quite at will by other means. 



The work on Echinarachnius, Arbacia, and Asterias was carried 

 out at Woods Hole in the summer of 1916 and that on Strong- 

 ylocentrotus and Orthasterias at Friday Harbor in 1917. For 

 materials and facilities at these stations the author is indebted to 

 Dr. F. K. I.illie and Dr. YV. C. Frye, and for many suggestions, 

 discussions, and even for generous contribution of experiment, il 

 data special thanks are extended to Dr. C. M. Child. 



EXPERIMENTAL. 



Physiological Axial Gradients. In scores of species <t plant * 

 and animals of all degrees of complexity of organization a gradient 



