136 NAOKI SUGITA 
changes from violet to blue, owing to the deeper staining of the 
Nissl bodies and of the intercellular tissue with the carbol- 
thionine. The apical dendrite thickens rapidly during the period 
in which the brain weight increases from 1.0 to 1.3 grams, but 
the basal dendrites are not clearly stained until the brain at- 
tains 1.6 grams in weight. Throughout the later life, the cyto- 
plasm is slowly but continuously decreasing in the absolute mass 
as the age advances, and the size of the nucleus is also diminish- 
ing. The nucleolus in the nucleus attains also its full size (the 
diameter is somewhat less than 2 micra) at the time when the 
nucleus has attained the maximum size, but it tends to grow 
slightly in late rages, while the nucleus show some decrease in size. 
The structure of the nucleus of the pyramids is not clearly 
demonstrable with this stain. As far as can be judged from the 
present preparations, the chromatin substance in the nucleus 
begins to develop notably only after the brain has attained the 
weight of 1.0 gram, and after the nucleus has passed its phase of 
rapid enlargement. 
From the foregoing it will be seen that up to a brain weight of 
0.95 gram, the pyramids may be regarded as in the preparatory 
stage of structural development, attaining at the end of this 
period nearly the full size of the cell body and of the nucleus. 
And after this stage increase and differentiation in the cytoplasm 
and the nucleus chromatin continue slowly until a brain weight 
of 1.1 to 1.3 grams. After that time they begin rather to di- 
minish in size, but nevertheless, to advance more and more in 
differentiation, which latter change probably indicates the matur- 
ing of the function of the pyramids. Morphologically, the 
pyramids first attain their fully mature aspects at a brain weight 
of about 1.6 grams (about 50 days in age). 
In my previous studies on the development of the cortex 
(Sugita, 717 a, 718 b), I named three phases of cortical growth in 
the early life of the albino rat; the first phase: from birth to the 
tenth day; the second phase: from the tenth to the twentieth 
day, and the third phase: from the twentieth to the ninetieth 
day. Applying this series of phases to the cytological develop- 
ment of the pyramids, the following appears. 
