128 D. H. TENNENT. 



•determined by experiment, to fertilize the eggs with normally 

 very active sperm. 



All attempts at cross-fertilization of the eggs immediately after 

 I their removal from the ovary were unsuccessful. 

 Jfoira d 

 ^°' ^^'^ Toxopneustes 9 ^'°''^' ^^^ ^^^^ ^^'^'"^ ^1^°^^'^^ ^^ ^^and 



u c c 1 , r , Moira d^ 



m sea water for hve hours and for the -r-, — : — ?v crosses for seven 



Arbacia V 



hours, before fertilization. 



Fully 95 per cent, of the eggs so treated underwent a regular 

 and comparatively uniform cleavage, the greater number devel- 

 oping into swimming blastulae and gastrulae. About 75 per 

 cent, of these embryos never developed beyond this stage. The 

 remaining 25 per cent, developed into plutei which remained 

 alive and were kept under observation for about ten days. 



This high percentage of segmentation was never approached 

 ia experiments in which chemicals were employed as aids in 

 effecting cross-fertilization, although naturally no attempts to 

 bring such methods to perfection were made after I had obtained 

 so simple a means of bringing about the results that I desired. 



The fact that the eggs were actually fertilized was recognized 

 in the transparent Toxopneustes eggs by the observation of the 

 union of the pronuclei. In the cases of both the Toxopneiistes 

 and Arbacia eggs a fertilization membrane was formed. 



Each series was checked by a control series of unfertilized 

 ■eggs. In these controls the eggs were allowed to stand, with 

 occasional changes of sea water, and in every instance the eggs 

 ultimately disintegrated without undergoing segmentation. 



The figures that illustrate the account that follows were drawn 

 from sections of picro-acetic and sublimate-acetic material stained 

 in iron ha.'matoxylin. 



A. The Moira d" Toxopneustes ? Cross. 



The Toxopneustes eggs stood in sea water, which was changed 

 four times, for five hours when they were fertilized with Moira 

 sperm. 



Cleavage began 40-45 minutes later. The time consumed 

 during the entrance of the spermatozoon, fusion of the pronu- 



