38 THE DEVELOPMENT OF THE CHICK 
great majority of animals. Harper observed that the number 
of sperm-nuclei formed in the pigeon varied from twelve to twenty- 
five in different cases. Only one of these serves as a functional 
sperm-nucleus; the remainder or supernumerary sperm-nuclei 
migrate, as though repelled, from the center towards the margins 
and deeper portions of the germinal disc, where they become 
temporarily active, dividing and furnishing a secondary area of 
small cells (accessory cleavage) surrounding the true cleavage- 
cells produced by division of the central portion of the dise around 
the descendants of the segmentation nucleus. It has been sup- 
posed by some authors who studied the selachi that the de- 
scendants of the supernumerary sperm-nuclei form functional 
nuclei of the so-called periblast, but this view has been disproved 
for the pigeon (Blount), in which it can be demonstrated that 
the supernumerary sperm-nuclei have but a brief period of 
activity, and then degenerate. 
Ill. CLEAVAGE OF THE OvuM 
The fertilized ovum is morphologically a single cell, with a 
single nucleus, the first segmentation nucleus. The living proto- 
plasm is aggregated in the germinal disc, and the remainder of 
the ovum is an inert mass of food material destined to be assimi- 
lated by the embryo which arises from the germinal disc. The 
first step in the development is a series of cell-divisions of the 
usual karyokinetic type, restricted to the germinal dise, which 
rapidly becomes multicellular. As the early divisions take place 
nearly synchronously in all the cells, there is a tendency for the 
number of the cells to increase in geometrical progression, fur- 
nishing 2-, 4-, 8-, and 16- ete., celled stages; but sooner or later 
the divisions cease to be synchronous. All of the cells of the 
body are derived from the germinal disc, and the nuclei of all 
cells trace their lineage back to the first segmentation nucleus. 
The supernumerary sperm-nuclei do not take part in the forma- 
tion of the embryo. 
Cell-division is the most conspicuous part of the early de- 
velopment; hence this period is known as the cleavage, or 
sezmentation, period. But it should be remembered first, that 
cell-division is as constant a process in later embryonic stages as 
in the cleavage period, and second, that it is probable, though 
little is known yet about this subject in the bird’s egg, that 
