284 



THE SEGMENTATION OF THE EGG. 



the principle of bipolarity with which the eggs of all animals 

 begin life. This is a principle which is evinced to the latest 

 period of the life of the adult, in the relation of back to front, or, 

 as it is more frequently called, above and below. 



There is one peculiar feature in the development of the egg- 

 stage, which I have not as yet said anything about. I refer to 

 the self-division of the egg. It is a well-demonstrated fact, that, 

 just before the organization proper begins to form, the egg 

 of all animals divides itself into a greater or less number of 

 minute spheres ; and in doing this it adopts a process which is 

 most remarkable as an instance of studied regularity in such a 

 simple and apparently formless body. 



There is an almost infinite number of varieties or modifica- 

 A B tions of this process ; but I cannot at 



present do more than display the essen- 

 tial features. I will take for my illus- 

 tration the egg of Laomedea, which I 

 have already described on a previous 

 occasion (p. 33, fig. 14). At first each 

 egg divides into two more or less dis- 

 tinct halves (fig. 187, A) ; and at the 

 moment this begins, the germinal vesicle 

 bursts, and its albuminous contents are diffused and mixed with 

 the surrounding material. As a second step, each half again 

 divides ; and thus is produced a quadruple body (B), instead of 

 the simple one of a few minutes before. Then each of the four 

 divides again ; by this increasing the number of the segments to 

 eight (C). After this it is difficult to follow the process of seg- 

 mentation, on account of the overlapping of the little spheres, 

 and their opacity ; but still we can readily observe that they are 



Fig. 187. The segmentation of the egg of Laomedea amphora. Ag. 125 

 diam. A, the first division into two masses; B, the second division into four 

 segment spheres ; C, the third division in eight masses, of which four (a) are 

 nearest the eye, and four (i) alternating with a, lie in the distance. D, the 

 so-called mulberry-mass, which results from the continued repetition of the self- 

 division of the segment spheres. Original. 



Fig. 187. 



