18 ArRtTiFIcIAL 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. 4 
Fics. 3 and 4.—Origin of the fertilization membrane threugh 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 
Fras. 5, 6, and 7.—-Further stages of the process of membrane formation. 
out on the surface of the egg.! 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) 
‘T have never observed this phenomenon in the eggs of Arbacia in Woods 
Hole. 
