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 far as 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 unsegmented ring of periblast. Second, accord- 

 ing to several observers, the center of the cleavage, i.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 



