120 J. Thomas Patterson. 



On account of the obliquity of the so-called horizontal clefts, the 

 central cells are characterized by great irregularity both in shape 

 and size. Many of them are shaped like squamous epithelium, and 

 often give the appearance of stratified epithelium in section. 



A detailed drawing of the anterior end of a section will show the 

 manner in which the marginal cells add their products to the central 

 area (Fig. 27). The cleavage membranes cut down deep into the 

 disc, and at their terminal points the horizontal clefts begin to spread 

 beneath these large cells, thus separating them from the underlying 

 j)ortion ; when this is accomplished, the large cells are split up into 

 smaller ones by vertical and horizontal divisions (on right of Fig. 

 27). 



XII. Ojs'e IIujxdeed and Fifty-four Cells in Surface View. — 



Seven Hours. 



During the next hour and a half the marginal cells undergo but 

 few divisions, but the central ones multiply very rapidly, and since 

 they receive but few additions from the marginal cells, their in- 

 crease must be due to their own activity in division. This results in 

 producing a large number of small cells in the central area, and the 

 smallest cells lie at the very center of the blastoderm (Fig. 30), It 

 would seem, therefore, that the early cleavage of the hen's egg fol- 

 lows the rule which states that the time occupied between any two 

 successive cleavages grows shorter and shorter as the volumes of the 

 cells decrease. 



The average depth of the central part of the blastoderm at this 

 period is about three cells (Fig. 31). The cleavage cavity, although 

 not so distinct as in the preceding stage, is, nevertheless, clearly 

 recognizable and there are no connections between its floor and the 

 lower cells of the blastoderm. The anterior and posterior ends of 

 the section show different conditions in the character of the cells. 

 At the posterior end there are three large cells, in addition to the 

 marginal one, which have not been broken up by horizontal clefts. 

 At the anterior end, on the other hand, all of the cells, except the 

 marginal, have undergone division. This difference is probably 

 only a local condition, and, therefore, is not fundamental. 



