72 THE MATURATION OF THE EGG OF THE MOUSE. 



PLATE 4. 

 Second Maturation Spindle (Figures 190-236, Inclusive). 



Figs. 17a, 176. The two sections show a spindle in a more advanced stage of divi- 

 sion than that in figs. 15a, 156. The abstriction of the polar cell has 

 begun in the vicinity of the "Zwischenkorperchen." Ovarian egg. 

 X (2500)2000. 



Fig. 18. Polar cell recently abstricted. Ovarian egg. X (2500)2000. 



Figs. 19a, 196. Two sections of an oviducal egg showing polar cell and egg nearly 

 severed from each other. Prophase of second spindle. X(2 5oo) 

 2000. 



Figs. 20, 21. Polar views of chromosomes of second spindle. Fig. 20 from an ovi- 

 ducal egg. Fig. 2 1 from an egg in periovarial space. X (2500)2000. 



Fig. 22. Side view of second spindle. Large first polar cell on nearly opposite side 

 of egg. Oviducal egg. X (1200)960. 



Figs. 23a, 236. Spindle in paratangential position, cut obliquely into two sections. 

 There are 19 chromosomes. Circumpolar bodies not stained deeply. 

 First polar cell very small and near the spindle. Oviducal egg. 

 X (2500)2000. 



PLATE 5. 



Second Spindle and Formation op Second Polar Cell. 



Figs. 24a, 246. A spindle similar to that of fig. 23, cut into two parts. There are 

 20 chromosomes. First polar cell absent. Oviducal egg. X(25oo) 

 2000. 



Figs. 25-27. Old second spindles from three eggs showing diminution of circum- 

 polar bodies. All from oviducal eggs without first polar cell. X(2 5oo) 

 2000. 



Figs. 28a, 28b. Polar views of the two daughter plates of a dividing second spindle 

 in a stage corresponding to that in fig. 16, plate 3. The first polar cell 

 is very small. Oviducal egg. X (2500) 2000. 



Figs. 29a, 29ft. Two sections of an oviducal egg. The oblique spindle is more ad- 

 vanced than the one in fig. 28. The stage of the abstriction of the 

 polar cell (see also fig. J, p. 40) corresponds to that of figs. 16-17. The 

 first polar cell is seen lying at the left of the second in fig. 29b. The 

 egg contains the heads of two spermatozoa. X(25oo) 2000. 



Fig. 30. Oviducal egg showing second polar cell newly abstricted, the first polar 

 cell, and the head of a spermatozoon. X (1200) 960. 



Figs. 31a, 316. Two sections of an egg (another section of which, less highly mag- 

 nified, is shown in fig. 40) exhibiting in fig. 31a the first polar cell 

 lying in the enlarged perivitelline space. X(i20o) 960. 



PLATE 6. 



Figs. 32-37. First polar cells from oviducal eggs which contain the second spindle. 

 They form a series of steps which illustrate the degeneration of the 

 first polar cell. Figs. 32 and 33 are of polar cells which have divided 

 into two or more parts. X (2500) 2000. 

 Figs. 38-40. Three stages in the process of ovulation. In all three cases the egg con- 

 tains the second spindle and is accompanied by the first polar cell. 

 X(i7o) 136. 

 Note. A minute body appearing in the clear space between zona pellu- 

 cida and vitellus in fig. 38 is due to a defect in the plate. 



PLATE A. 



Fig. A. Mouse cage. For description see p. 6. 



Figs. B, C. Suspended mouse cages, with self-recording apparatus to indicate ap- 

 proximately the time of parturition of a gravid female. For descrip- 

 tion see pp. 7-10. 



Fig. G. Chromosomes of first maturation spindle. See pp. 28-30. 



(Plate A faces page 6.) 



