The Morphology and Physiology of the Nucleolus. 143 
energy is liberated and can be used by the cell for other purposes 
as described. 
Nakahara (44) believes that the nucleolar substance extruded 
from the nucleus of the silk-gland cells of insects gives off 
phosphorus and forms a lower member of the nuclei series. 
Transformation of nucleolar extrusions into albumen granules is 
described by the same writer in adipose cells of insects undergoing 
active metabolism ; and Schreiner (51) has described the forma- 
tion of fat globules from, nucleolar extrusions in myxine. 
The critical point in these cases is whether it is the nucleolar 
substance itself which is altered chemically, or whether it is the 
cytoplasm in which the enzyme had diffused. If it is the 
nucleolar substance itself that is so changed chemically, then 
either the enzyme must pass out of the nucleus, or be present in 
the cytoplasm ; in that case the nucleolus represents an accumula- 
tion of products of metabolism within the nucleus, either secretory 
or excretory. On the other hand, if the nucleolar extrusions are 
not themselves transformed chemically, then they would seem to 
be of the nature of enzymes liberated from the nucleus, and the 
nucleolus would be an important organ of nutrition. In those 
cases of nucleolar extrusion described in Limntea , the extruded 
nucleolar material seems in all cases to dissolve and diffuse in the 
protoplasm. 
(e) Extrusions from the Nucleus into the Cytoplasm during Cell 
Division . — Quite apart from the extrusion of nucleolar material 
from the nucleus of the resting cell, it is a significant fact that 
when cell division takes place, the whole of the nucleolus with the 
exception of the nucleolinus appears to disintegrate and a part 
probably diffuses into the cytoplasm. In all cases of mitosis, 
wdaich have been observed in my material, no nucleolus was 
discernible during metaphases or anaphases, but it disappeared at 
prophases and reappeared at late telophases. 
As is seen at Plate Y, the nucleolus of embryonic cells is by 
no means small in comparison with the size of the cell, so that at 
each mitosis there is possibly some nucleolar material together 
with karyolymph extruded into the cytoplasm. The possibility 
of elimination of nucleolar material from the nucleus during 
mitosis, offers a likely explanation of the comparative rarity of 
nucleolar extrusions during embryogeny when it might be expected 
to occur most often. It seems that in the newly formed embryonic 
•cell, the chromosomes spread out to form the nuclear reticulum in 
order to exert their maximum influence during metabolism. The 
nuclear membrane is formed, and as microdissection has shown (48) 
this is of a gel-like consistency, and no doubt exerts a selective 
influence, determining what substances shall pass into, and out of 
the nucleus during the “ resting stage.” Material passing into the 
nucleus is utilized for building up chromatin to restore the normal 
