The Morphology and Physiology of the Nucleolus. 
133 
tliis cell possibly denotes that the nucleus has arisen by fusion of 
two nuclei such as are seen in fig. 10. Similar fusion of nuclei 
has been observed in tissue cultures. 
The Nucleolus during Fertilization. 
The nucleus of the mature ovum contains little or no oxyphil 
nucleolar substance, and no definite nucleolar substance can be 
observed during the formation of the polar bodies. In fig. 12 is 
shown a sperm nucleus (H) which has penetrated the oocyte. No 
nucleolus is visible, and the sperm-head stains faintly basophil. 
The division aster is seen in process of formation at A. A 
noteworthy feature of the ovum at this stage is the vacuolation of 
its cytoplasm. Similar vacuolation has been seen by other 
observers in eggs of various animals. 
A later stage in fertilization is shown in fig. 13. Here the 
first polar body (P x ) has been formed and is beginning to 
disintegrate ; the second polar body (P 2 ) has, been just formed, and 
the female pronucleus (FP) is moving away towards the centre of 
the cell. At SN is seen the sperm nucleus. In both male and 
female pronuclei, the chromosomes are somewhat vesicular, and no 
definite nucleolus is discernible. The sperm aster (A) is seen 
passing towards the centre of the cell, where fusion of the pronuclei 
takes place. 
Both male and female pronuclei stain very badly, which renders 
accurate observation most difficult. 
The Nucleolus during Early Segmentation Stages. 
A section of the four-cell stage is shown in fig. 14. The remains 
of the polar bodies are represented at P x and P 2 . The two micro- 
meres are in process of division. Attention is drawn here to the 
absence of anything but yolk granules (Y) in the cytoplasm during 
cell division, in preparation fixed in corrosive acetic. In the 
megamere, mitochondria (M) and Golgi elements (GA) are shown 
as they appear in osmic acid preparations. The appearance of the 
so-called “ resting ” stage of the nucleus during these early stages 
is very remarkable. At fig. 15 is shown the nucleus of an eight- 
een stage. A faint nuclear reticulum is distinguishable, and 
scattered about it are numerous very faintly stained globules, some 
of them stained slightly oxyphil (OY), others slightly basophil (BY). 
At NX is shown what appears to be an outflow of the faintly 
stained oxyphil fluid. A similar structure has been observed in 
numerous cells at similar stages of development, and in no case has 
a definitely formed nucleolus been seen. 
