GERMINAL MEMBRANE. 53 
applied to a drop of the contents of the egg, it does not appear 
to act materially upon the cells, and the contained corpuscle 
becomes paler and somewhat swollen, which could not well 
take place if it were fat. These cells, then, are the earlier 
stage of development of the subsequent globules of the yelk- 
cavity. The larger ones already resemble them perfectly. 
These globules of the yelk-cavity are therefore likewise cells. 
Their nucleus-globule (Kernkugel) is acted on by acetic acid 
precisely in the same way as it was in the earlier condition. 
It does not lie centrally in the cell, but on the internal surface 
of the wall, as is seen when the cells are caused to roll under 
the microscope. When at rest, however, they are generally so 
placed that the nucleus-globule occupies the most depending 
point (because probably it is the heaviest portion of the cell), and, 
on that account, it then appears to lie in the centre of the cell. 
The yelk in the first instance contains only the yelk-cavity, 
with its cells; the proper yelk-substance with its globules not 
being as yet formed. The colour of these young eggs is there- 
fore also white, like the contents of the yelk-cavity. 
The membrane-like layer which surrounds the above-described 
contents of the egg, may be completely separated from the parts 
which surround it externally with facility, after the egg has 
been divided through the centre. It is not connected with 
them, and appears, to the unaided eye at least, to be pretty 
smooth on its external surface ; it is not possible to trace it 
towards the interior. Its structure is peculiar. Purkinje, who 
discovered it, describes it as consisting of globules, which re- 
semble in form and size, but are more transparent than the 
blood-corpuscles. When spread out upon a plate of glass, and 
examined with the microscope, it is seen to consist of two parts, 
an internal minutely granulous stratum, and an external layer 
of cells. Numerous little granules are observed in the internal 
stratum, which resemble the nuclei of the above-described cells 
of the yelk-cavity in their earliest stage, and I conjecture that 
the cells of the yelk-cavity are formed from this stratum, so 
that in fact it still pertaims to the yelk-cavity. The external 
layer consists of small round granulous cells, each of which 
contains a nucleus, which again in many instances encloses 
one or two nucleoli. ‘Two or three such layers of cells lie one 
above another. ‘These layers of cells are surrounded externally 
