DEVELOPMENT PRIOR TO LAYING 41 



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 set of cleavage planes 

 are vertical like the preceding planes, but they tend to be variable 

 otherwise. In Fig. 16 C there is shown an eight-celled stage in 

 which three of the new furrows are approximately at right 

 angles to the second cleavage plane, but other arrangements are 

 found. 



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 peril^last, 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 sometimes excentric. 

 It was believed by those who emphasized this point, that the 

 displacement is towards the posterior end of the blastoderm; but 

 Coste, for instance, failed to note any excentricity, and Patterson 

 noticed both conditions, and showed that the displacement might 

 even be towards the anterior end of the blastoderm. In the 

 pigeon, according to Miss Blount's observations recorded below, 

 excentricity appears to be exceptional; moreover, 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 interpreta- 

 tion 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, l)ut these are, at first, still 

 connected below l)y the deeper protoplasm. They may be called 

 the central cells. These are bounded by cells that are united 

 in the marginal periblast, and thus lack marginal boundaries as 



