The Morplioloyy and Physiology of the Nucleolus. 147 
nucleolus becomes differentiated into two parts, one still stains oxyphil, the 
other basophil (fig. 3). 
5. Throughout oogenesis, the oxyphil nucleolus appears to extrude 
portions of itself into the cytoplasm ; this process is most active towards the 
end of oogenesis, when the oxyphil nucleolus becomes vacuolated and 
breaks up into numerous small nucleoli, most of which pass out into the 
cytoplasm (figs. 4, 5, 6). 
6. The basophil staining part of the nucleolus persists until the end of 
oogenesis, when it breaks up and forms a granular substance, which 
becomes evenly distributed throughout the nucleus, and in fixed material 
appears as small accumulations of basophil material arranged upon the 
nodes of the nuclear reticulum (figs. 4, 5, 6). 
7. No definite nucleolus is visible in the nuclei of the polar bodies or in 
the female pronucleus (fig. 13). 
B. During Spermatogenesis. 
1. An oxyphil staining nucleolus is present in the spermatocyte. There 
is no basophil staining nucleolus, and no differentiation into two parts as 
occurs in the oocyte (fig. 7). 
2. The nucleolus persists in the spermatocyte until the synizesis stage 
of the reduction division. When the chromosome threadwork undergoes 
contraction, the nucleolus is drawn into the contracted chromosome reti- 
culum (fig. 7). During diakinesis no nucleolus is distinguishable, but the 
chromosomes stain less basophil than they did during synizesis. 
3. No nucleolus was discernible in the spermatid during its metamor- 
phosis into the spermatozoon (fig. 9). 
C. In the Sertoli Cells of the Ovatestis. 
1. The influence of the sperm cells upon the sertoli, or “ nurse -cells,” is 
such that the nucleolus of the “nurse-cell” nucleus is characteristically 
enlarged (figs. 8, 9, 10, 11). 
-2. The “ nurse-cells ” attached to the developing spermatozoa are often 
binucleate, and a large nucleolus is usually present in each nucleus (fig. 10). 
3. When there is only one nucleus in the “nurse-cell,” it is usually 
elongated and contains two nucleoli, one at each end of the nucleus (fig. 9). 
4. There is a probability of nucleolar extrusion taking place in the 
“nurse-cell” after the sperms have become detached from it (fig. 11). 
D. During Fertilization. 
Nucleoli are either altogether absent from the pronuclei, or else they 
are achromatic, for none can be seen in the pronuclei before, or for some 
time after their fusion (fig. 13). 
E. During Early Segmentation Stages. 
1. During cell division the chromosomes have their normal appearance. 
No granules other than yolk bodies and cytoplasmic organs have been 
observed in the cytoplasm, and there is no evidence of any germ-cell deter- 
minant derived from the nucleolus or chromatin (fig. 14). 
2. The nucleus of the “ resting cell ” presents a remarkable appearance. 
On a just distinguishable nuclear reticulum, numerous globules are to be 
seen, some stain faintly basophil, others slightly oxyphil. There is some 
evidence for believing that the latter may be extruded into the cytoplasm 
fig. 15). 
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