THE EARLY DEVELOPMENT OF THE CHICK 243 



protoplasm merges with a well-marked region of the white 

 yolk called the nucleus of Pander, which connects with the 

 central white yolk by a narrow stalk called the latebra (Fig. 

 85, A). In the disc itself two regions may be distinguished, a 

 large central area, which is to form the blastoderm proper, and 

 a narrow marginal area of denser appearance, known as the 

 periblast. Peripherally the periblast continues for some dis- 

 tance, perhaps completely, over the surface of the vitellus as 

 an extremely thin protoplasmic layer. 



The first cleavage furrow appears as a short shallow groove, 

 near the middle of the germ disc, in length approximately one- 

 half the diameter of the disc (Fig. 89, A). Sections show that 

 this cleavage also fails to extend completely through the disc 

 vertically (Fig. 90, A). It is not known that the first cleavage 

 plane coincides with the median sagittal plane of the embryo: 

 indeed it seems probable that, as in other eggs of this extremely 

 meroblastic type, there is no correspondence between these 

 two planes. The position of the main embryonic axis is, how- 

 ever, fairly uniform, though not completely fixed. It lies 

 approximately at right angles to the long axis of the whole egg, 

 the anterior end of the embryo directed to the left, when the 

 sharper end of the egg is held pointing away from the observer. 

 In the few cases actually observed, the first cleavage plane 

 seems to have no definite relation to these axes. 



The second cleavage is also vertical, and approximately at 

 right angles to the first, giving four adequal cells, all still in- 

 complete (Fig. 89, B) . About an hour after the first cleavage, 

 the third appears. Typically, though not in a majority of 

 instances, this is a fairly regular cleavage, parallel with the 

 first, and dividing the disc into two rows of four cells each. 

 Frequently this cleavage is more or less irregular in form, and 

 the synchronism of division is lost by this time (Fig. 89, C) . 

 The subsequent stage, consequently, may be said to consist 

 only approximately of sixteen cells. These are very irregular, 

 but the general tendency of the fourth plane is to separate each 

 of the eight cells into a central and a distal cell. 



During the appearance of the third and fourth cleavages, 



