Plate I. 



Fig. 1. Tbe mature ovum in its mucilaginous coat at the moment of impregnation. One sperma- 

 tozoon is entering the yolk and the vitelline membrane is just beginning to separate. 

 Diameter of the egg including its coat: 0,i6 mm. 



» 2. The same ovum slightly later. The vitelline membrane has separated and the egg shows 

 very fine radiating pseudopodia. 



J) 3. The same ovum at about 10 minutes after the fecundation. The male and female pro- 

 nuclei have fused to form the first segmentation nucleus, round which the clear plasm 

 shows a radiate arrangement. The vitellus is surrounded by a thin layer of homoge- 

 neous plasm, which is omitted in most of the figures, as is the case also with the mu- 

 cilaginous coat. 



» 4. The same ovum somewhat older. The nucleus has begun to change into a spindle and 

 traces of a radiate arrangement of the plasm are still seen at its poles. 



» 5. The same ovum slightly later. The spindle is complete and a plasmatic radiation at each 

 pole becomes discernible. 



)i 6. The same ovum at about 30 minutes after the fecundation. The spindle and the radia- 

 tions begin to disappear. 



» 7. The same ovum slightly later, showing the pestle-shaped figure. 



y) 8. The same ovum somewhat older. The polar radiations have begun again to appear. 



B 9. The same ovum at about 40 minutes after the fecundation. The first division has begun. 



» 10. The same ovum slightly later. The two segments are separated, oval and slightly flattened 

 against each other. The radiations remain distinct. 



» 11. The same ovum slightly later. The segments have changed into spheres. The radiations 

 have disappeared. 



7) 12. The same ovum slightly later. The segments have changed into hemispheres and become 

 pressed against each other. The nuclei are well defined. A radiation is traceable. 



» 13, The same ovum slightly later. The segments have begun to separate again. The nuclei 

 are indistinct and the radiation has disappeared again. 



» 14. The same ovum slightly later. The segments have separated, are oval again and show 

 the pestles. 



» 15. The same ovum at about one hour and 10 minutes after the fecundation. 



» 16. The same ovum 5 — 10 minutes later. The second segmentation stage is accomplished, four 

 equpl segments having separated. Seen from above. 



» 17. The same ovum slightly later. The segments begin to press against each other. 



p 18. The same ovum slightly later. 



» 19. The same ovum slightly later. The segments are closely pressed against each other. 

 Seen from above. 



» 20, A spermatozoon. Length: 0,o6 mm. 



N. B. Where no measurements are given, the figures are more or less highly magnified. 



