EMBRYOGENESIS FROM THE SPERMATOZOON 305 
of a small amount of some substance having a low refractive 
index about the middle pieces of some of the spermatozoa. In 
favorable cases as many as 60 per cent of the spermatozoa may 
undergo this change. At this time many of these spermatozoa 
are still swimming. During the course of the next few hours 
these lowly refractive areas increase in size until they are about 
half as long as the sperm head and acquire a fairly distinct 
ellipsoidal outline. Then in many cases the sperm head can 
be seen to be bent in a horseshoe or spiral shape, and to be 
included in the wall of the vesicle, which has now become 
spherical, while the tail of the spermatozoon still remains 
unchanged or has disappeared without taking any part in the 
transformation. The next change is an increasing indistinct- 
ness in the sperm head, and an increasing refractive power of 
the whole vesicle so that it can hardly be discriminated at all 
in the albumen. It is not possible to follow the process farther 
in unstained material. 
In some cases these vesicles instead of being spherical stretch 
out along the whole side of the sperm head, or may become 
entirely disconnected from the spermatozoon. 
If yolk is used as a culture medium for the sperm essentially 
the same phenomena occur; and in the various Ringer solutions 
vesicles containing the sperm heads are also formed, but in the 
Ringer solution, as a rule, the steps in the formation of these 
vesicles could not be seen without staining. 
When the hanging drops are fixed in Flemming’s fluid and 
stained and examined in Herla’s vesuvin and malachite-green 
mixture, it can be seen that in its early stages the vesicle has 
distinct walls and a homogeneous unstained fluid of a low 
refractive index in its interior. This fluid is possibly water and 
this would account for the fact that the vesicle is conspicuous 
in albumen and yolk, but invisible in Ringer solution. 
The vesicle seems to be formed by the imbibition of water 
by the very thin protoplasmic envelope of the sperm head and 
