vm] SEGMENTATION 117 



types of fertilisation are all differences of detail. In essen- 

 tials the process of fertilisation is uniform throughout the 

 animal kingdom; it consists in the coming together of two 

 nuclei derived from the two parents, and the mingling and 

 division of their substance in such a way that the chromo- 

 somes from each parent are equally represented in both 

 the daughter nuclei produced by the first segmentation 

 division. 



Some discussion of the relation of fertilisation to the 

 initiation of development in the egg will be given in con- 

 nection with the subject of Artificial Parthenogenesis in a 

 later chapter. It will be convenient, however, here to 

 devote a few lines to the early segmentation divisions which 

 normally follow immediately after the conjugation of the 

 sperm and egg nuclei. The zygote nucleus as has been said, 

 divides immediately, but the nature of the division of the 

 cell varies greatly according to whether the egg is poor or 

 rich in yolk. In an egg with relatively little yolk, as for 

 example in Mammals, Amphibia, Echinoderms and many 

 other Invertebrates, a complete cell-division takes place 

 immediately after the nuclear division, separating the egg 

 into two blastomeres which are only loosely connected with 

 each other. These two blastomeres are usually of equal 

 size, though in some forms one is large, containing most of 

 the yolk, and the other small. The position of the first 

 cleavage plane nearly always has some definite relation to 

 the orientation of the parts of the future embryo, and most 

 frequently corresponds with its sagittal plane, dividing the 

 egg into right and left halves. It is evidently of importance, 

 therefore, to discover what determines the position of this 

 first cleavage plane, and whether it depends on the structure 

 of the egg alone, or may be affected by the spermatozoon. 

 The same problem arises in eggs in which the cleavage is 



