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 disc around 

 the descendants of the segmentation nucleus. It has been sup- 

 posed by some authors who studied the selachii 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. 



III. 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 disc, 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- etc., 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 

 segmentation, 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 



