384 JACQUES LOEB AND F. W. BANCROFT 
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 middle 
piece. For after the formation of the vesicle, head, tail and 
middle piece are, so far as can be seen, unchanged (fig. 1). In 
’ many cases the vesicle is seen at the front end of the spermatozo6n. 
Such cases result from the bending of the spermatozo6n in the 
middle piece region, as the series illustrated in figs. 1, 2 and 3 
show. The maximum development of the unchanged vesicle is 
shown in fig. 4. 
When the vesicle has reached its full size the material of which 
its surface is composed seems to wet the sperm head very easily. 
For in the next stage the sperm head is in contact with the wall 
of the vesicle along its whole length, and the vesicle has usually 
assumed a more or less spherical shape (figs. 5 to 8). 
Up to this point the transformations were found to take place 
in the same way in all the media employed, but in the various 
Ringer solutions the transformation went no farther than this 
even when the spermatozoa were left in the solutions for forty- 
eight hours and longer. 
In the yolk and albumen, however, the development toward the 
formation of a nucleus went a little farther. In these media solu- 
tion of the sperm head took place, which seemed to begin as soon 
as the head was drawn into the vesicle. For in these media stages 
like figs. 7, a, c, d, e, were quite difficult to find, while in the Ringer 
solutions such stages, and those in which the sperm head within 
the vesicle was entirely unchanged were the most frequent trans- 
formations observed (figs. 5 and 6). 
In preparations that were fixed after the spermatozoa had been 
in contact with the yolk or albumen only two or three hours the 
most frequent transformation of the vesicles observed is one in 
which the head has entirely disappeared while the whole vesicle 
takes a rather dilute nuclear stain. In a few cases nuclei of this 
type still show remnants of the sperm head as in fig. 7, 6 and c. 
These appearances would seem to indicate a solution of the chro- 
matin of the sperm head in the contents of the vesicle. 
