DEVELOPMENT PRIOR TO LAYING 43 
Duval interprets a narrow space observed by him between the single 
superficial layer of cells and the deeper cells of the germinal dise as the 
segmentation cavity; it is thus entirely distinct from the subgerminal 
cavity which arises much later, according to his conception. Apart 
from the fact that his figures appear to represent the merely virtual 
space between the superficial cells and the underlying cells in an exag- 
gerated form, the interpretation appears to me to be incorrect. It is 
based on the theory that the deeper cells represent the primary entoderm, 
a view which I cannot accept; the interpretation of this space as cleavage 
ravity fails if it be shown (see beyond) that the underlying cells are not 
entoderm. 
The account given above of the deeper cleavages, those seen in 
section, is the conventional one, based on the observations of Kolliker, 
Duval, and others. The account, that follows, of the corresponding 
cleavages in the pigeon’s egg, is different in some important respects, 
that bring it into agreement with the best known meroblastic eggs, 
those of the bony fishes. I have, however, allowed the above account 
to stand, though I consider it probable that a careful re-examination 
would bring the cleavage of the hen’s egg into line with that of the pigeon 
and the teleost. 
The Pigeon’s Egg. The cleavage of the pigeon’s egg has 
been worked out in more detail than that of the hen’s egg (Blount) ; 
as it offers some interesting features that have never been de- 
scribed for the hen’s egg, and must be made the basis of the 
description of the formation of the germinal wall and the germ- 
layers in the absence of any consistent account for the hen’s egg, 
it will next be described. The fundamental features of the cleav- 
age are the same as in the hen’s egg, so that the description need 
not be repeated. 
The feature to be particularly emphasized in the cleavage 
of the pigeon’s egg is the occurrence of a secondary or accessory 
cleavage in the marginal zone or periblast (Figs. 17 and 18 A). 
When the origin of these cells is traced it is found that they arise 
around the supernumerary sperm-nuclei, which accumulate and 
multiply in the periblast. The complete history of these nuclei 
has been worked out by Harper and Blount, so that there 
‘an be no doubt as to their derivation. Another interesting 
point illustrated by the figures is that the marginal cells have 
a peripheral wall wherever the accessory cleavage occurs, but 
between the groups of accessory cleavage cells the marginal cells 
are continuous with the periblast (Figs. 17 and 18 A), as they are 
