36 



THE STRUCTURE OF THE FERTILISED EGG 



Fig. 12. Diagram of the cleavage in a sea urchin egg under pressure. 

 (a-c) normal cleavage (4, 8, and 16-cell stages); id-i) cleavage in 

 an egg flattened by external pressure; (d-f) seen from the side; 

 ig-i) seen from above. Analogous nuclei marked in the same way 

 in all cases. After Diirken. 



eggs with a hair ligature, giving them a dumb-bell shape. The 

 nucleus lay in one end of the dumb-bell shaped egg, and only 

 this part was able to start cleaving. The other, non-nucleated 

 half remained uncleaved (Fig. 13 a, b). After a certain number 

 of divisions, however, when the nucleated part had divided into 

 8 or 16 cells, one of the cleavage nuclei would pass from this 

 half, through the ligatured peduncle, into the other part 

 (so-called "retarded nucleation" of this half), (Fig. 13 c). 

 This half then began to cleave as well. If at this stage the 

 two halves were entirely separated, each would, under certain 

 circumstances, develop into a normal embryo (Fig. 13 d). 

 Therefore, the nucleus which passed through the peduncle, and 

 which represents only one eighth or one sixteenth of the 

 original fertilisation nucleus, is sufficient to bring about normal 



