PROCESS OF SEGMENTATION 



17 



the long axis of the mitotic spindle. Figure 5 shows the succes- 

 sion of the cleavage divisions in the egg of the pigeon. The 

 diagrams represent surface views of the blastodisc and an area 

 of the surrounding yolk, the shell and albumen having been 

 removed. The observer is looking directly at the animal pole. 

 Figure 5, A, should be compared with Figure 4. The diagrams 

 of Figure 4 are of sections cut in a plane which passes vertically 

 through the blastodisc and which is at right angles to the plane 

 of the first cleavage (Fig. 5, A, 1-1). The first cleavage furrow 

 cuts into the egg in a plane coinciding with the imaginary axis 

 passing through the animal pole and the vegetative pole. The 

 two daughter cells or blastomeres resulting from the first 

 cleavage are not completely walled off but each remains 

 unseparated from the underlying yolk (Fig. 4). 



In each of the two blastomeres resulting from the first cleav- 

 age division, mitotic spindles initiating the second cleavage arise 

 at right angles to the position which was occupied by the first 

 cleavage spindle. This determines that the two second cleav- 

 age furrows will be at right angles to the first. Since these 

 two second cleavage furrows lie in the same plane and are 

 apparently continuous they are usually considered together. 

 They mark the position of the second cleavage plane which cuts 

 the egg in the animal-vegetative axis but which lies at right 

 angles to the first cleavage plane (Fig. 5, B, 11-11). A very 

 good way of getting a clear conception of the orientation of the 

 cleavage planes is to cut them in an apple. Let the core of the 

 apple represent the animal- vegetative axis of the egg. The first 

 cleavage furrow can be represented by notching the apple 

 lengthwise, that is as one ordinarily starts to split an apple into 

 halves. The second cleavage furrow can be represented by 

 cutting into the apple again in a plane passing through the 

 axis of the core, but at right angles to the first cut, as one would 

 start to quarter the apple. 



The third cleavage furrows are variable in number and in 

 position. In the most typical cases each of the four blastomeres 

 established by the first two cleavages divides again so that eight 

 blastomeres are formed (Fig. 5, C). Frequently, however, the 

 third cleavage appears at first in only two of the blastomeres, 

 so that six cells result instead of eight. 



The fourth series of cleavages takes place in such a manner 



