STUDIES OF FERTILIZATION | 371 
About thirty-seven minutes after insemination (metaphase 
of first maturation division) the sperm is still readily found on 
the exterior of the vitelline membrane external to the substance 
of the entrance cone which is now lens shaped. The substance 
of the entrance cone is homogeneous and it stains less than before; 
it is sharply marked off from the unaltered egg cytoplasm by a 
layer of small basophile granules. In the center of its external 
face is a sharply differentiated granule which stains intensely 
black in iron haematoxylin, and which is connected to the sperm 
head by a fine thread passing through the vitelline membrane; 
penetration has already begun. 
Forty-three minutes after insemination (late metaphase of 
the first maturation division) the entrance cone sinks into the 
egg-cytoplasm, and the head of the spermatozoon begins to be 
drawn within the egg in the form of a thick thread, less than one- 
third the diameter of the sperm head, however. The sperm 
nucleus is being drawn through the small perforation in the vitel- 
line membrane. 
Forty-eight minutes after insemination (stages of anaphase 
of the first maturation division), nearly all of the sperm head is 
drawn into the egg in the form of a thick thread several times 
longer than the original sperm head. Before the head is entirely 
within the egg its inner end begins to swell and becomes vesicular. 
The entire entrance cone penetrates the egg-protoplasm always 
retaining its original connection with the apex of the spermato- 
zoon, so that the original orientation of the sperm is preserved 
and may be readily recognized after penetration. 
Fifty-four minutes after insemination (telophase of the first 
maturation division), the entire head of the spermatozoon is 
within the egg. The tail and middle piece usually remain without. 
As I intend to publish a separate account of the interesting 
details of penetration of the spermatozoon, and as the later stages 
do not concern the present problem, I shall simply say, there- 
fore, that as the united sperm-head and entrance cone penetrate 
farther into the egg cytoplasm, they rotate in such a way that the 
entrance cone which was originally in advance of the sperm nu- 
cleus comes to lie behind it. During the rotation the sperm 
