ON DEVELOPMENT OF AT YE PH IRA COMPRESSA. 403 
into the yolk or vitellus.” I believed for a long time that the 
nutritive elements were derived from the vacuolar spaces already 
mentioned, because in these spaces I have often observed, in 
fresh specimens, small refracting spheres which resemble very 
much those of the yolk-elements. I have, however, found out 
that my so imagined yolk-spheres were nothing but the 
particles of water that have, in some way or other, found their 
way into these vacuolar spaces; for when I examined the ovary 
in the solution of bicarbonate of potash or ammonia (2 per 
cent, strong), I have invariably found that the vacuolar spaces 
are free from any such thing, and that these elements are only 
seen when the ovary is examined under water. 
As the egg increases in size the protoplasm becomes coarsely 
granular at the periphery. These granules are quite opaque 
at first, but become more or less transparent as they grow in 
size ; they assume a deep greenish colour and become con- 
verted into the yolk-spheres. The yolk-spheres are in general 
round and homogeneous, showing no trace of a nuclear struc- 
ture. Their size varies much, the smallest being no larger 
than yolk-granules, and the largest often measure *003 mm. in 
diameter. My observations therefore point out that the yolk- 
spheres originate endogenously out of the protoplasm of the 
egg- 
This agrees with the statements made by Huxley in his 
Crayfish (-MpO; and still more with the results obtained by 
Professor Whitman in his Clepsine 
As the yolk-spheres and the vacuoles are formed the proto- 
plasm of the egg becomes reticular in its structure, suspending 
these spherules in its meshes. But at two places the proto- 
plasm remains in a more or less thick continuous layer. These 
are around the germinal vesicle, and at the extreme periphery* 
of the egg. The peripheral protoplasm appears to give origin 
to a distinct membrane, which I call by the indifferent name of 
“ primary egg-membrane.” In the oviduct it receives another 
membrane from the lining epithelial cells, which at this time 
present a glandular appearance (fig. 15). This outer membrane 
will be spoken of as “ secondary egg-membrane.” Between 
