EXPLANATION OF PLATES. 



All drawings were made with the aid of a camera lucida. The figures as repro- 

 duced are four-fifths the diameter of the original drawings. The magnification 

 appended to the description of each figure is that of the reduced reproduction, the 

 magnification of the original drawing being in parenthesis. 



The magnification of 2500 diameters (reduced = 2000) was obtained with a 

 Zeiss 2mm. homog. immersion apochromatic objective and No. 12 compensating 

 ocular; that of 1200 (reduced = 960) , with 2mm. objective and No. 6 compensat- 

 ing ocular; that of 880 (reduced = 704), with 2mm. objective and No. 4 compen- 

 sating ocular; and that of 170 (reduced = 136), with Zeiss A objective and No. 4 

 Huyghenian eyepiece. 



PLATE 1. 



Origin of First Maturation Spindle. 



Fig. 1 . Germinative vesicle shortly before the disappearance of its nucleolus and 

 the transformation of its contents into the fundaments of the chromo- 

 somes and the spindle fibers. Ovarian egg. X (2500) 2000. 



Fig. 2. Early stage in the formation of the chromosome fundaments. Ovarian 

 egg. X(25oo) 2000. 



Figs. 2a, 2b. Fundaments of chromosomes in sections adjacent to that of fig. 2. 



Figs. T,a, 3&. Two consecutive sections showing a somewhat later stage than the 

 preceding. Ovarian egg. X (2500) 2000. 



Figs. 4, 4a. Chromosomes (20 in number) more completely differentiated. Spindle 

 not yet formed. Nuclear membrane still intact. Ovarian egg. 

 X (2500)2000. 



Fig. 5. Section of a young spindle showing faint, fibrillations. There are 20 chro- 

 mosomes scattered over its surface. Nuclear membrane is dissolved 

 at some points. Ovarian egg. X (2500)2000. 



Fig. 6. Composite drawing of a spindle cut into three parts. There are 20 chromo- 

 somes. Stage slightly more advanced than that illustrated in fig. 5. 

 Nuclear membrane completely vanished. Ovarian egg. X (2500)2000. 



Fig s - 7. 7- Two consecutive sections of a spindle, like that shown in fig. 6, seen in 

 end view. There are 20 chromosomes, 10 in each section. The cyto- 

 plasm shows faint radiations about the spindle. Ovarian egg. 

 X (2500)2000. 



PLATE 2. 



First Maturation Spindle. 



Fig. 8. Ovarian egg. The chromosomes have become arranged in the plane of 

 the equator. X (880) 704. 



Figs. 8a, 86. Enlarged views of the two sections into which the spindle in fig. 8 is 

 cut. There are 20 chromosomes. X(25oo) 2000. 



Fig. 9. Section of a spindle like that in fig. 8. X (2500) 2000. 



Figs. - 1 oa, 1 ob. The two sections of a spindle of which the fibers at one pole converge 

 to a point. There are 20 chromosomes. Ovarian egg. X (2500)2000. 



Fig. 11. Section of a spindle similar to the preceding. Ovarian egg. X (2500)2000. 



Fig. 12. Ovarian egg. The polar ends of the spindle fibers are becoming thickened, 

 and the clear region about the spindle is visible. One of the 20 chro- 

 mosomes (some of which are in adjacent sections) has been displaced 

 into the cytoplasm. X (1200) 960. 



Fig. 13. Ovarian egg. The circumpolar bodies are formed at the poles of the spin- 

 dle, and the clear region is evident. X (1200) 960. 



Fig. 13a. More highly magnified view of the spindle shown in fig. 13. X(25oo) 2000. 



Fig- I 3^- View of that portion of the spindle seen in fig. 13a which falls in the fol- 

 lowing section. X(25oo) 2000. 



PLATE 3. 



Division of First Spindle and Abstriction of First Polar Cell (Figures 



14 to 18, Inclusive). 



Fig. 14. Ovarian egg containing an oblique spindle. Several of the chromosomes 

 have already divided. Circumpolar bodies numerous and conspic- 

 uous. X (1200)960. 



Fig. 14a. One chromosome from the spindle in fig. 14. 



Figs. 15a, 156. An oblique spindle in two consecutive sections, showing the mi- 

 gration of the daughter chromosomes. Ovarian egg. X (2500) 2000. 



Figs. i6a-i6d. Four consecutive sections of a spindle similar in stage of division to 

 that of fig. 17. See fig. H (p. 34). Ovarian egg. X(25oo) 2000. 



7i 



