DEVELOPMENT PRIOR TO LAYING 41 
line (Fig. 16 B), or they may meet the first cleavage furrow at 
separate points, in which case the intervening part of the first 
furrow becomes bent at an angle, forming a cross furrow. The 
third cleavage of the hen’s egg has not been figured or described 
by any author, so faras I know. But it is probable from analogy 
with other similar forms of cleavage that in each of the four 
cells a furrow arises approximately at right angles to the second 
furrow and parallel to the first, thus producing eight cells in 
two parallel rows of four each. But the variable forms of the 
succeeding cleavage stages indicate a probable considerable 
variation in the eight-celled stage. 
Before describing the later cleavage stages, we should note 
certain important relations of the first four or eight cells: First, 
these are not complete cells in the sense that they are separate 
from one another. They are, indeed, areas with separate nuclei 
marked out by cleavage furrows in a continuous mass of proto- 
plasm. The furrows do not cut through the entire depth of 
the germinal disc, and the cells are therefore connected below 
by the deeper layer of the protoplasm; nor do the furrows extend 
into the periblast, and all the cells are therefore united at their 
margins by the unseginented ring of periblast. Second, accord- 
ing to several observers, the center of the cleavage, 7.e., the place 
where the first two cleavage furrows cross, is excentric. It is 
believed by those who emphasize this point, that the displace- 
ment is towards the posterior end of the blastoderm; but Coste, 
for instance, failed to note any excentricity. The number of 
observations is still too few to admit of a safe conclusion on this 
point; in the pigeon, according to Miss Blount’s observations 
recorded below, excentricity appears to be exceptional; more- 
over, the excentric area may bear any relation whatever to the 
future hind end of the embryo, so that in the pigeon it will not 
bear the interpretation that has been placed on it in the hen’s 
egg. 
The following cleavages (after the eight-celled stage) in the 
hen’s egg are very irregular, but two classes of furrows may be 
distinguished in surface view: (1) those that cut off the inner 
ends of the cells, and (2) those that run in a radial direction. 
The furrows of the first class produce a group of cells that are 
bounded on all sides in surface view, but these are, at first, still 
connected below by the deeper protoplasm. They may be called 
