48 THE DEVELOPMENT OF THE CHICK 
is no way of timing the development, so that one has to judge 
the sequence of the stages, all of which come from the uterus, by 
the degree of formation of the shell, by the size of the cells and 
by the appearance of the sections. This can be at best only 
approximate; and, as the securing of any given stage is largely 
a matter of chance, no one has, as a matter of fact, secured a 
complete series. In the pigeon, on the other hand, the time 
since laying the first egg is a fairly exact criterion of the stage 
of development of the second egg. It has, therefore, been pos- 
sible to secure a complete series, and the subject has been worked 
out by Miss Blount, whose preliminary communication in Vol. 
XII of the Biological Bulletin furnishes the basis of the following 
account. 
The periblast ring is entirely devoid of nuclei after the super- 
numerary sperm-nuclei have degenerated. The marginal cells 
become greatly reduced in size owing to multiplication and > 
continuous production of central cells, and their nuclei thus 
approach more and more closely to the periblastie ring. The 
scene then changes; the marginal cells cease to produce central 
cells; when their nuclei divide the peripheral daughter-nuclei 
move out into the periblast, which is thus converted into a nu- 
cleated syncytium. The periblastic nuclei multiply rapidly and 
invade all portions of the periblastic ring, which maintains its 
original connection with the white yolk. Not only do the peri- 
blastic nuclei invade the periblastic ring, but some of them also 
migrate centrally into the protoplasm forming the floor of the 
segmentation cavity. They do not, however, reach the center, 
but leave a non-nucleated sub-germinal area, corresponding 
approximately to the nucleus of Pander, free from nuclei. The 
subgerminal syncytium may be known as the central periblast 
to distinguish it from the marginal periblast. They are, of 
course, continuous. In sections one has the appearance of nuclei 
in the yolk, for there is no sharp boundary between periblast 
and yolk (Fig. 22). The syncytium, which has received its nuclei 
from the marginal cells, is the primordium of the germ-wall (Figs. 
21, 22, 23, 24). 
There is a snarp contrast between the segmented blastoderm 
and the syneytial periblast not only in structure but also as 
regards fate. The marginal cells constitute a zone of junction be- 
tween blastoderm and periblast. Thus in Fig. 22 it will be ob- 
