EFFECTS ON EGGS OF FRESH WATER PULMONATES 421 
The first polar body is extruded about two hours after the egg 
is laid. At the moment of its extrusion the outline of the entire 
egg becomes irregular and a ^^yolk lobe" frequently, if not in- 
variably, appears at the vegetal pole. The second polar body is 
formed about one hour after the first; during its formation the 
clear protoplasm is spread out as a cap at the animal pole, and no 
deep ''well" of such protoplasm is found as during the first matur- 
ation (figs. 4, 5). The egg again becomes irregular in outline at 
the moment the polar body is extruded and a yolk lobe is formed. 
The appearance of the living egg before and during these matura- 
tion divisions suggests the view that during the nuclear division 
the surface tension of the egg is increased, giving rise to what 
Reinke (1900) has called ''mitotic pressure;" and that this ten- 
sion is reduced the moment the polar body begins to be extruded. 
During and immediately after the second maturation division 
the cap of clear protoplasm at the animal pole begins to spread 
over the upper hemisphere (figs. 5-8), and at the time of the first 
cleavage it has reached the equator of the egg (fig. 9, 10). The 
advancing edge of this cap becomes thickened, so that in optical 
sections it frequently has the appearance of extending nearly 
through the egg (fig. 11), as described by Mark. 
When this clear cap has extended about half way to the equa- 
tor, the sperm nucleus may be seen approaching the egg nucleus 
from the lower pole, both appearing as clear spots in the egg. 
They meet at the animal pole, where chey He in a deeper portion 
of the clear protoplasm. Immediately beneath these nuclei is a 
layer of finely granular yellow substance, which later surrounds 
the nuclei and the cleavage spindle (figs. 7, 8). From its position 
relative to the nuclei and spindle there can be little doubt that 
this substance corresponds to the " archoplasm" of Ascaris (Boveri 
1887), or the "sphere material" of Crepidula (Conklin 1902). 
The germ nuclei then appear to elongate, the wall between the 
two nuclei disappears and the two asters of the first cleavage 
spindle become visible as clear areas at the ends of the first cleav- 
age spindle. Even after the central portion of the spindle has 
ceased to be visible the asters may still be seen as clear areas in 
the egg. 
