THE PROCESS OF CLEAVAGE. 63 



ivliereas in the case of the Frog's egg the first plane of cleavage cuts 

 through to the opposite pole, in the case of the Hen's egg it divides 

 only the germ-disc into two similar segments, which like two buds 

 rest upon the undivided yolk-mass with a broad base, by means of 

 which they still have a physical connection with each other. Soon after 

 this, there is formed a second vertical furrow, which crosses the first 

 at right angles, and likewise remains limited to the germ-disc, which 

 is now divided into four segments (fig. 33 }. 



Each of the four segments is again divided into halves by a radial 



furrow. The segments thus formed correspond to sectors, which 



meet in the centre of the germ-disc with pointed ends, and have 



a- 5 



it? / 



e 



fig. 34. Section through the germ-disc of the Hen's egg during the later stages of segmentation 



after BAI.FOUR. 

 The section, which represents rather-more than half the breadth of the blastoderm (the middle 



line is at c), shows that the segments of the surface and of the centre of the disc are smaller 



than those below and toward the periphery. At the border they are still very large. One of 



the latter is indicated at a. 

 a, Large peripheral cell ; b, larger cells of the lower layers ; c, middle line of the blastoderm : 



e, boundary between the blastoderm and the white yolk, ic. 



their broad ends turned toward the periphery. The apex of each of 

 the segments is then cut oft' by a cross furrow, i.e., by one which is 

 parallel to the equator of the egg (fig. 33 C), in consequence of which 

 there are formed smaller central (c) and larger peripheral (d) seg- 

 ments. Since from this time forward radial furrows and those that 

 are parallel to the equator make their appearance alternately, the germ- 

 disc is subdivided into more and more numerous segments, which are 

 so arranged that the smaller lie at the centre of the disc, therefore 

 immediately around the animal pole, the larger toward its periphery. 

 With the advancing cleavage the smaller segments are entirely con- 

 stricted off from the underlying yolk, whereas the larger peripheral 

 ones still rem.ain at first in continuity with it (fig. 34). In this way 

 we finally get a disc of small embryonic cells, which, toward the 

 middle, are arranged in several superposed layers. 



