112 
J. T. CUNNINGHAM. 
the region at which the polar disc is formed. Nearly the 
whole of the handle of the mallet is occupied by the interme- 
diate layer, and its surface is the parapolar region. The ovum 
assumes the ovoid form by the gradual shortening of the 
handle of the mallet and the consequent decrease in extent of 
the parapolar region. 
When the polar disc first appears it has some affinity for 
carmine : it exhibits a striation perpendicular to its surface, 
and contains none of the deuteroplasmic elements. After a time 
in the polar disc two layers can be distinguished : the super- 
ficial is not stained by picrocarmine. This achromophilous 
layer collects towards the centi’e of the disc, and there the 
chromophilous layer thins out, and finally disappears. Mean- 
time the vitelline membrane forms as an extremely thin layer 
all over the ovum, including the polar disc, but not over the 
achromophilous layer in its centre. This mass remains bare, 
occupying an aperture in the vitelline membrane — the micro- 
pyle, from which it usually projects slightly. This projecting 
mass of clear protoplasm van Beneden calls the plug of im- 
pregnation. 
The vitellus contains deuteroplasmic elements of three kinds, 
which can be distinguished after treatment with osmic acid 
and picrocarmine. They are (1) hyaline spheres, (2) homo- 
geneous droplets, and (3) refringent corpuscles. 
The hyaline spheres are definite in outline, and slightly 
tinged with the carmine ; they show no structure ; within them 
are usually smaller spheres like vacuoles. 
The homogeneous droplets are of indefinite outline and 
irregular shape, and are probably spaces in the protoplasm of 
the vitellus, filled with fluid or semifluid substance. 
The refringent corpuscles are definite in outline, somewhat 
angular in shape, and highly refractive. They are often 
arranged in series, and contribute to give the vitellus a radiate 
striation, the series converging towards the centre of the 
ovum. 
The protoplasm forms a continuous thin external layer round 
the vitellus, and fills up the spaces between the formed elements. 
