414 FRANK R. LILLIE 
it is often arranged in two masses as shown in fig. 1 with some 
karyolymph; sometimes the arrangement is more reticular, but 
the karyolymph is always in evidence in good preparations. The 
middle-piece is very distinctly set off from the head;.it is ring- 
shaped, very broadly attached to the base of the head. The 
attachment of the tail is to the margin of the ring, and is there- 
fore excentric to the axis of the head. There is a decided asym- 
metry, particularly illustrated in ), c, d, of fig. 1. 
2. The ovum 
The ovum has been described in the preceding parts (Lillie, 
711) of this study. <A brief description will therefore serve here. 
It is about 100u in greatest diameter, somewhat flattened in a 
polar direction, and girdled by a double zone of large oil drops 
embedded in the yolk-bearing protoplasm (text fig. 1). It is 
bounded by a vitelline membrane and possesses a coarsely alveolar 
cortical layer about 7u in thickness external to the yolk-bearing 
layer. : 
3. Observations on fertilization in the living egg 
If the eggs remain unfertilized in the sea-water, maturation 
does not take place, and the egg remains unchanged with intact 
germinal vesicle. 
a. The cortical changes. When insemination takes place a 
large number of spermatozoa become attached to the ovum if 
the sperm is present in excess. In about two to three minutes 
all spermatozoa, with the exception of one, which is alone con- 
cerned in the subsequent fertilization, begin to be carried away 
from the surface of the egg by an outflow of jelly from the ovum. 
This is more particularly described in the first of these studies 
(F. R. Lillie,’11). The jelly is formed from the alveolar contents 
of the cortical layer, which gradually disappears, until in the 
course of about fifteen minutes the original cortical layer is repre- 
sented only by the perivitelline space and the delicate walls of | 
the original alveoli crossing this space to the plasma membrane 
(see study 1). 
