DEVELOPMENT OF THE FROG S EGG. 



295 



around Ithaca and also Schenectady, N. Y. These were collected 

 early in the morning, as soon as possible after laying. They 

 were brought into the laboratory, divested of their jelly, and 

 rayed while still in the undivided or two-celled stage. A thin 

 tin-foil diaphragm perforated by a hole three tenths of a milli- 

 meter in diameter shielded most of the surface area of the eggs 

 from the rays but permitted a relatively small portion of that 

 area to be influenced. After raying, the eggs were transferred 

 to a jar containing 1,000 c.c. of tap water in which they were 

 permitted to develop, the water being changed frequently. 



Physicists have definitely established the fact that ultra-violet 

 light rays possess a very slight penetration capacity. We are 

 justified, hence, in assuming that the chemical alterations pro- 

 duced by the rays acting under these conditions were restricted 

 to a superficial layer of protoplasm of the egg in the restricted 

 area mentioned. The interpretations presented in this paper 

 are made, accordingly, upon that basis. 



The surface area of the egg subjected to examination in this 

 experiment is best demonstrated by a reference to Fig. I. A 



FIG. i. 



i 

 two-cell ovum is shown with the animal pole uppermost. The 



broken line marks out the area investigated. As is to be seen 

 this extends from the region of the equator up towards the animal 

 pole but is exclusive both of the pole itself and of a narrow median 

 strip which connects the latter with the equator. As can be 

 seen this region is exclusive of the gray crescent. Different 

 portions of this area were rayed successively with the three 

 tenths millimeter beam of light. As a result, when development 



