THE EAHLY DEVELOPMENT OF THE MARSUFIALIA. 127 



thin shell-membrane externally (s.m.) (about '0016 mm. in thickness), 

 the albumen (rdh.), zona (z.p.)- smd the deutoplasmic (d. z.) and formative 

 (/. z.) zones of its cytoplasmic body. The male proniicleus is visible in 

 the formative zone. Diameter of entire egg about '29 mm. (D. viv., 

 15. 19 . vii . "01. f. Picro-nitro-osmic and iron-hgematoxylin.) 



Fig. 11. — Photo-micrograph ( x 15t.») of section of unsegmented ovum 

 from the uterus, slightly older than that of fig. 10. Diameter of entire 

 egg in fresh state "Slf-'So.mm.. of the ovum proper "3 x "28 mm. ; thick- 

 ness of shell, '0024 mm. In the figure the female pronucleus is visible 

 near the centre of the formative zone (/. z.), and the male pronucleus 

 lies a little above it and to the right. The periviteUine space (j)-^-) 

 is partially occupied by coagulum. (D. viv., 21 . v . '03, ^. Hermann, 

 iron-hgenuitoxylin.) 



PLATE 2. 



Fig. 12. — Photo-micrograph ( x 150) of an unsegmented ovum from 

 the uterus, of the same batch as that of fig. 11, and 'o-l mm. in diameter. 

 The two pronuclei are visible in the central region of the formative 

 zone. 



Fig. 13. — Photo-micrograph ( x 330) of uterine ovum. Stage of first 

 cleavage spindle. Diameter, "315 mm. (D. viv.. 1, 15 . vii . '01. |. 

 Picro-nitro-osmic, iron-ha^matoxylin.) 



Fig. 14. — Photo-micrograph ( X about 78) of egg in the 2-celled stage, 

 taken immediately after its transference to the fixing fluid. Lateral 

 view. y. b. Yolk body. Diameter of entire egg about '34 mm. (D. viv., 

 1, 15 . vii . '01. Picro-nitro-osmic.) 



Fig. 15. — Photo-micrograph ( x about 78) of another 2-celled egg, 

 seen from lower jDole. Diameter, "35 mm. (D. viv., 4 B, 23 . vi . '02. 

 Perenyi's fluid.) 



Fig. 16.— Photo-micrograph ( X about 78) of another 2-celled egg, 

 of the same batch as preceding. End view, showing one of the two 

 blastomeres and the yolk-body {y. b.). 



Fig. 17. — Photo-micrograph (x 150) of vertical section of 2-celled 

 egg. '34 mm. in diametei-, showing the shell-membrane (•0064 mm. thick), 

 traces only of the albumen, the zona (z. j:).), and the two blastomeres (the 

 left one measuring, from the sections, "16 x "18 x '10 mm., its nucleus 

 •031 X '027 mm. ; the right one, -16 x "19 X "09 mm., its nucleus, '03 x 

 •028 mm.). Note the differentiation in their cytoplasmic bodies. 

 (D. viv., 6, 21 . vii . '01, f. Picro-nitro-osmic and iron-ha^matoxylin.) 



Fig. 18. — Photo-micrograph (x 150) of vertical section of 2-celled 

 egg, "32 mm. in diameter, with shell-membrane "005 mm. thick, showing 

 the two blastomeres, and enclosed between their upper ends the yolk- 



