18 AKTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



more distinctly followed in the egg of Strongylocentrotus pur- 

 puratus by lowering the temperature of the sea-water; by this 

 means, the process of membrane formation is retarded, and it 



Fig. 3 



Fig. 4 



FIGS. 3 and 4. Origin of the fertilization membrane through the formation 

 of small vesicles on the surface of the egg. 



can be observed in its separate phases. Since we shall see later 

 that this process of membrane formation is the essential part 

 in the activation of the egg, we will trace it through its separate 

 stages. 



The beginning of the process shows itself in a roughening 

 of the hitherto smooth surface of the egg (Fig. 3). This is 

 due to the formation of countless tiny vesicles which stand 



Fig. 5 Fig. 6 Fig. 7 



FIGS. 5, 6, and 7. Further stages of the process of membrane formation. 



out on the surface of the egg. 1 These small droplets quickly 

 increase in size (through absorption of sea-water) and flow 

 together into larger drops (Fig. 4). This goes on (Figs. 5, 6, 7) 



1 1 have never observed this phenomenon in the eggs of Arbacia in Woods 

 Hole. 



