DEVELOPMENT PRIOR TO LAYING 47 
but beyond the zone of the accessory cleavage there may occur 
two or three concentric circles variously indicated (lig. 17). 
Vacuoles, appearing black in the photographs, are very common 
in the outer zones. These appearances indicate that the peri- 
blastic protoplasm extends farther out in the superficial white 
yolk than is usually believed to be the case; and this suggests an 
interesting comparison with the teleost ovum, where the peri- 
blastic protoplasm surrounds the entire yolk as a very thin layer. 
Sections confirm the idea that the periblastic protoplasm has an 
extension beyond the so-called margin of the blastodise. Some 
eggs show a more definite margin than others; it may be that 
there is a periodic heaping of the periblast at the margins, for 
which again an analogy may be found in teleosts. 
Although the smallest cells may be more or less excentric in 
the segmented germinal dise of the pigeon, their position bears 
no constant relation to the future embryenic axis. They may 
lie in this axis in front of or behind the middle, or to the right or 
left of it (ef. Fig. 18 A-D). 
At the eight-celled stage a horizontal cleavage plane begins to 
appear beneath the central cells (ig. 19). This marks the full 
depth of the blastoderm at all stages, and the several-layered 
condition arises by horizontal cleavages between this and the 
surface. Comparison of Figs. 19, 20, and 22, drawn at the same 
magnification, will show that the depth does not increase by addi- 
tion of cells cut off from below, as is usually supposed to be the 
‘ase in the bird’s ovum. The first horizontal cleavage plane not 
only marks the full depth of the blastoderm, but it also indicates 
the site of the segmentation cavity which arises gradually by aceu- 
mulation of fluid between the cells and the underlying unseg- 
mented protoplasm and yolk. The segmentation cavity gradually 
extends towards the margin of the blastoderm, but it is bounded 
peripherally by the zone of junction between the marginal cells 
and the periblast. 
IV. ORIGIN OF THE PERIBLASTIC NUCLEI, FORMATION OF THE 
GERM-—W ALL 
Our knowledge of this part of the subject in the hen’s egg is 
very incomplete, and the various accounts are contradictory. 
The reason for this is the great difficulty of securing a complete 
series of stages, and of arranging them in proper sequence. There 
