161 
Erik Nordenskiold 
plasma has also the same arrangement of its chief components: the outer 
layer has the same fibrillar structure as previously described in the sperma¬ 
tocytes; it is however much thicker in comparison with the inner layer; the 
latter also shows the same granular substance and the same aggregations of 
mitochondria around the nucleus. 
The further development of the spermatid begins, as usual, with a change 
in the position of the nucleus which moves through the plasma to the peri¬ 
phery and becomes closely attached to the cell membrane (PI. XI, fig. 5). 
Here it must be observed that the final position of the nucleus depends on 
that of the spindle of the last division: the nucleus always passes to the side 
where the remains of the spindle appear as an appendix to the plasma (PI. XI, 
fig. 6). This circumstance makes it possible to follow the different constituents 
of the spermatid during its subsequent intricate process of metamorphosis. 
The above-mentioned change of place is the signal for a series of modifica¬ 
tions in the shape of the nucleus. At first its constitution is altered: its several 
components, linin, chromatin and nucleolus seem to be effaced and to emerge 
as a homogeneous, uniformly colourable matter. This substance shrinks and 
becomes concentrated towards the inner part of the nuclear vesicle (PI. XI, 
fig. 5), the outer part being occupied only by nuclear fluid; later on the nuclear 
membrane shrinks round the substance, thus considerably diminishing the 
volume of the nucleus. There now begins a still more important change in 
the shape of the nucleus: first it becomes somewhat pear-shaped, later on it 
elongates and becomes rod-shaped (PI. XI, figs. 6 to 12). In transverse sections 
this nuclear rod (PI. XI, fig. 9) shows a peculiar structure, owing to the disposal 
of the chromatin in peripheral longitudinal filaments surrounding a central 
chromatin cord. In order to define the orientation of the rod, the exterior 
end, which is turned towards the fragment of the spindle, will be termed 
hereafter the external end, and the opposite end will be called the internal end. 
During this nuclear process important changes are taking place in the 
plasma: (1) the endoplasma. Here the mitochondria, which, as is shown above, 
were crowded around the nucleus in the form of several well-circumscribed 
masses, are dispersed homogeneously through the whole endoplasma (PL XI, 
figs. 4, 5), and at this stage it is impossible to discern them as a special 
ingredient of this matter. Through this dissolution of the mitochondria the 
endoplasma itself acquires a greater affinity to stains, which characterizes it 
during the following stages. Round the external end of the nucleus the 
endoplasma penetrates through the ectoplasma and here appears at the 
surface of the cell (PI. XI, figs. 8, 10, 11). At the opposite side it also expands 
and forms a very characteristic process, at first obtusely conical (Fig. 8), 
which later on comes to play a very important part in the development. 
Even more remarkable are the changes in (2) the ectoplasma. The original 
fibrillar structure of the plasma-zone disappears; the fibrils melt away and 
simultaneously the whole layer increases considerably in thickness. In the 
now nearly homogeneous layer, appear several large vesicles, distinguishable 
