154 MR A. ANSTRUTHER LAWSON ON 
vacuole in which the chromatin reticulum lies suspended in the karyolymph. This 
enlargement is not so great as that which occurs at a similar time in the heterotype 
mitosis, but nevertheless sufficient to make these nuclei very conspicuous. ‘The 
chromatin now changes from the finely divided reticulate condition to long, rather 
stout threads, and ultimately into the compact definite chromosomes, as shown in 
fig. 40. This figure represents a section of a somatic cell taken at a time when the 
chromosomes are nearly organised. The nuclear vacuole, it will be seen, is perfectly 
spherical and obviously under high osmotic pressure. It should be noted in passing 
that, unlike sporogenous cells, there are several large vacuoles in the cytoplasm. 
In the following stages, namely, those represented in figs. 41, 42, 43, and 44, I was 
able to confirm the main observations of Niimec (1899) and others in regard to the 
development of the two polar caps at opposite sides of the nucleus. The beginning 
of these kinoplasmic caps is shown in fig. 41, where they appear in section as 
shallow crescent-shaped groups of threads. As indicated in figs. 42, 43, and 44, these 
crescent-shaped caps appear to elongate and become decidedly conical in form. These 
features do not differ essentially from what is commonly known in this connection. 
Unfortunately, however, a very interesting and important fact has been overlooked. 
It is, namely, that as these kinoplasmic structures known as the polar caps develop, 
there is a corresponding diminution in the volume of the nuclear vacuole. This is 
so clearly illustrated in figs. 41, 42, 48, 44, and 45, that actual measurements 
are not necessary. ‘The decrease in the amount of karyolymph is gradual but 
unmistakable. It will also be noticed from these figures that as the karyolymph 
diminishes the outline of the nucleus becomes less spherical. As shown in figs. 48, 44, 
and 45, the nuclear vacuole becomes decidedly flattened on the sides that form the 
bases of the polar caps. 
There is no doubt whatever in my mind that the factors responsible for the forma- 
tion of the polar caps are the same as those which we have described above as being 
concerned in the formation of the kinoplasm during the heterotype mitosis. They are, 
namely, that a state of tension has been created in the cytoplasm by the reduction 
in the volume of the nuclear vacuole, and the cytoplasm thus finds itself obliged to 
occupy a greatly increased cubical space. ‘I'his tension finds an expression in the 
drawing out of threads from the cytoplasmic reticulum by the receding nuclear 
membrane. 
It is curious that somatic mitosis should be characterised by the drawing out of 
only two conical-shaped sheaves of kinoplasm. ‘This feature, however, becomes less 
dificult to understand when we remember that the numerous vacuoles which are 
always present in the cytoplasm would render a radial or a multipolar arrangement 
of the kinoplasm impossible. In sporogenous cells these cytoplasmic vacuoles do 
not occur. 
The developmental stages that now follow are practically the same as those | 
described above for the heterotype spindle. ‘The diffusion of the karyolymph continues 
