422 
W. A. HASWELL. 
The older eggs are to be recognised by the eyes, which are 
visible through the shell. These may be placed in such a 
way as to show that the young Temnocephala is lying with 
its long axis parallel with that of the egg, but in a large 
number of cases the position is a transverse one, and occa- 
sionally an intermediate condition occurs. This variation in 
the direction of the long axis is not due to movement of the 
larva in its later stages; the direction varies from the outset, 
and regulating the direction of sectioning, except in advanced 
embryos, is little more than mere guesswork. 
After the ovum has become fertilised, and the egg com- 
pleted by the addition of the mass of yolk-cells and the 
enclosing shell, it appears to pass without much delay into 
the genital atrium, which serves the purpose of a uterus. 
Here it may remain some little time before passing out 
through the genital aperture ; but the stage of development 
which has been attained when the deposition takes place 
varies. In most cases an egg from the ootype or atrium 
contains an ovum in which the process of segmentation has 
not yet begun ; and unsegmented ova are occasionally found 
among those attached to the surface of the crayfish ; but 
occasionally segmentation is found to be well advanced in a 
uterine egg. 1 have never found more than one egg in the 
uterus. 
When the egg is fully formed, the greater part of its mass 
consists of yolk-cells. These are polyhedral cells of an aver- 
age diameter of about '050 mm., with granular contents. 
Each has a nucleus '015 mm. in long diameter, of oval or 
elliptical shape, with a single spherical nucleolus with a dia- 
meter of about '005 mm. The yolk-cells are at first quite 
distinct from one another (PI. 23, fig. 2) ; but, as develop- 
ment proceeds, a gradual coalescence takes place, beginning 
at the periphery, and eventually (PI. 24, fig. 7) the entire 
mass completely fuses to form a syncytium, in Avhich all 
trace of cell outlines has become completely lost. When the 
formation of the syncytium has begun, the nuclei of the more 
superficial cells pass outwards and come to lie close to the 
