134 
Transactions of the Society. 
The Nucleolus during the Blastula and 
Gastrula Stages. 
By the time that a typical blastula stage is reached a well- 
marked oxyphil staining nucleolus is present in each cell. Fig. 16 
shows a portion of a blastula wall. Two of the cells (CF X , CF 2 ) 
have recently divided, and small nucleoli have just formed amongst 
the disappearing chromosomes (CH). In the cell below is seen a 
large nucleolus protruding from the side of the nucleus. Such an 
appearance is fairly common. Within the other cell, mitochondria 
(M) and Golgi elements (GA) are shown as they appear in 
osmic acid preparations. Part of another blastula is figured in 
Plate V, fig. 17. In the lowest cell, the chromosomes (CH) are 
seen forming upon the nuclear reticulum, preparatory to cell 
division. The nucleolus is shrunken in size, and instead of stain- 
ing markedly oxyphil, stains slightly basophil. It is interesting 
to note that here again the change in staining reaction of the 
nucleolus takes place at the time when changes are occurring in 
the chromosomes. In the cell above, the chromosomes (CH) are 
grouped equatorially upon the spindle. In this metaphase, as in 
the previous cases, there is no sign of the nucleolus within the 
cytoplasm. The cell BC is remarkable in that it is binucleate; 
such a condition has been observed in several blastulae at this 
period of development. In the cell BC each part of the nucleus 
has a nucleolus, but this is not always the case, as is seen by 
referring to fig. 18<x, where another double nucleus is drawn. 
Here the larger part of the nucleus has a quite big nucleolus, but 
Explanation of Plate IV. 
Fig. 10. — Binucleate nurse cell with developing spermatozoa (H) attached. 
Each part of the nucleus has a large vacuolated nucleolus (NL). 
Fig. 11. — Cell which has functioned as a nurse cell for spermatozoa with three 
still attached to it. The nucleus is curiously lobed, and has two large nucleolar 
masses, one of which (NX) appears to be in the act of extrusion into the cytoplasm. 
Fig. 12. — Ovum in which the sperm (H) has just penetrated. The aster is 
seen forming at A. The cytoplasm contains numerous yolk granules and is con- 
siderably vacuolated (V). 
Fig. 13. — Ovum, showing the polar bodies (Pj and P 2 ), and the male (SN) and 
female pronuclei (FP). The division aster is shown at A. This figure has been 
drawn by superimposing drawings of four consecutive sections. 
Fig. 14. — Section of an early segmentation stage. Pj and P 2 are the remains 
of the polar bodies. The two micromeres are at the metaphase of cell division. 
The megamere is shown as it appears at this stage with Kopsch’s osmic acid 
technique. 
Fig. 15. — “ Resting nucleus of early segmentation stage. The nucleus con- 
tains numerous' globules, some staining oxyphil (OV), others basophil (BV). The 
nuclear reticulum is scarcely distinguishable, and there appears to be an absence 
of basophil staining substance in the nucleus. At NX is shown what seems to be 
an extrusion of the faintly staining oxyphil substance. 
Fig. 16. — Portion of the wall of a blastula, showing the characteristic oxyphil 
staining nucleolus. The cells CFj and CF 2 have just undergone mitosis. 
