MorpHoutoGy oF DEVELOPMENT 19 
until finally the contents of all the drops have run together 
into a continuous layer around the egg! (Fig. 2). Hence the 
surface lamellae of the tiny droplets form later the fertilization 
membrane. 
At higher temperatures the process of membrane formation 
in freshly removed eggs usually proceeds so quickly that the 
stages depicted in Figs. 3 to 7 are not distinctly seen and the egg 
passes directly from the condition of Fig. 1 to that of Fig. 2. 
At first the membrane adheres closely to the egg, and then of a 
sudden the space between the cytoplasm and the membrane 
increases enormously. In this latter way the process of mem- 
brane formation occurs in the egg of Arbacia. 
After the formation of this fertilization membrane a 
second change takes place in the surface of the egg, inasmuch as 
a gelatinous film or membrane (G.M. Fig. 8) gradually appears 
on the surface of the cytoplasm. This 
gelatinous film does not form as sud- Pin om 
denly or quickly as the fertilization 
membrane, but only begins to appear 
after ten minutes or more. The writer 
considers it possible that the formation 
of this film depends upon processes of 
oxidations, since its formation is de- 
layed if the oxidations in the egg are Fro. 8—Formation of a 
+ ti G.M. aroun 
retarded by the presence of KCN. che prataniaemnifirtlio Geet 
: about 15 minutes after the 
After membrane formation, the formation of the fertilization 
chemical processes which underlie de- geen ae 
velopment set in in the egg. The nuclear material grows and 
nuclear division occurs; this nuclear division, or rather the 
so-called nuclear spindle, is visible in the egg which we have 
chosen for description, viz., that of Strongylocentrotus purpuratus 
(Fig. 9). The spindle formation visible in this figure is imme- 
diately followed by cleavage or cell division, i.e., the partition 
1 So-called ‘‘perivitelline space.” 
