52 Animal Life and Intelligence. 



Then sets in what is known as the segmentation or 

 cleavage of the ovum. First the nucleus and then the cell 

 itself divides into two equal halves (B.), each of these 

 shortly afterwards again dividing 1 into two. We may call 

 the points of intersection of these two planes of division the 

 " poles," and the planes " vertical planes." We thus have 

 four cells produced by two vertical planes (<7.). The next 

 plane of division is equatorial, midway between the poles. 

 By this plane the four cells are subdivided into eight (D.). 

 Then follow two more vertical planes intermediate between 

 the first two. By them the eight cells are divided into 

 sixteen. These are succeeded by two more horizontal 

 planes midway between the equator and the poles. Thus 

 we get thirty-two cells. So the process continues until, by 

 fresh vertical and horizontal planes of division, the ovum is 

 divided into a great number of cells. 



But meanwhile a cavity has formed in the midst of the 

 ovum. This makes its appearance at about the eight-cell 

 stage, the eight cells not quite meeting in the centre of the 

 ovum. The central cavity so formed is thus surrounded 

 by a single layer of cells, and it remains as a single layer 

 throughout the process of segmentation, so that there 

 results a hollow vesicle composed of a membrane constituted 

 by a single layer of cells (E.). 



The cells on one side of the vesicle are rather larger 

 than the others, and the next step in the process is the 

 apparent pushing in of this part of the hollow sphere ; just 

 as one might take a hollow squash indiarubber ball, and 

 push in one side so as to form a hollow, two-layered cup (F.). 

 The vesicle, then, is converted into a cup, the mouth of 

 which gradually closes in and becomes smaller, while the 

 cup itself elongates (G.).* Thus a hollow, two-layered, 

 stumpy, worm-like embryo is produced, the outer layer of 



* In some forms of life the opening of the cup marks the position of the 

 future mouth ; in others, of the future vent. In yet others it elongates into 

 a slit, occupying the whole length of the embryo ; the middle part of the slit 

 closes up, and the opeuiug at the far ends mark the position, the one of the 

 future mouth, the other of the future vent. 



