1 6 EARLY EMBRYOLOGY OF THE CHICK 



cytoplasm is aggregated. When, however, the yolk mass is 

 encountered, the process is greatly retarded. So slowly, in 

 fact, is the division of the yolk accomplished, thut succeeding 

 cell divisions begin at the animal pole of the egg before the first 

 cleavage is completed at the vegetative pole. 



The eggs of birds are also telolecithal, but the amount of 

 yolk which they contain is both relatively and actually much 

 greater than that in Amphibian eggs. Cleavage in bird's eggs 

 begins as it does in the eggs of Amphibia, but the mass of the 

 inert yolk material in them is so great that the yolk is not 

 * divided. ' The process of segmentation is limited to the small 

 disc of protoplasm lying on the surface of the yolk at the animal 

 pole, and is for this reason referred to as discoidal cleavage 

 (Fig. 5). The fact that the whole egg is not divided is indicated 

 • by designating the process as partial (meroblastic) cleavage 

 ^ in distinction to the complete cleavage (holoblastic) seen in 

 eggs containing less yolk. The cells formed in the process of 

 segmentation are known as blastomeres whether they are com- 

 pletely separated as results in holobastic cleavage or only 

 partially separated as results in meroblastic cleavage. 



The Unsegmented Blastodisc.^ — In the egg of a bird which is 

 about to undergo cleavage, the disc of active protoplasm at the 

 animal pole (blastodisc) is a whitish, circular area about three 

 millimeters in diameter. The central portion of the blastodisc 

 is surrounded by a somewhat darker appearing marginal area 

 known as the periblast. The protoplasm of the blastodisc, 

 especially in the periblast region, blends into the underlying 

 white yolk so that it is difficult to make out any line of demarca- 

 tion between the two. It is in the central area of the blasto- 

 disc that cleavage furrows first appear. Neither the nuclei 

 resulting from the early cleavages nor the cleavage furrows 

 invade the marginal periblast until very late in the process of 

 segmentation. 



The Sequence and Orientation of the Cleavage Divisions in 

 Birds. — The nature of the series of divisions in the meroblastic, 

 discoidal cleavage characteristic of the eggs of birds is largely 

 determined by the amount and distribution of the yolk. An- 

 other determining factor is the tendency of mitotic spindles to 

 develop so that the long axis of the spindle lies at right angles 

 to the axis of least dimension of the mass of unmodified cyto- 

 plasm. The cleavage furrow always arises at right angles to 



