EMBRYOLOGY 377 



The two nuclei lose their membranes, and the spiremes segment 

 to form the chromosomes, which arrange themselves on the equa- 

 torial line of the spindle, sometimes even remaining in two distinct 

 groups. Each of the pro-nuclei contains the haploid number of 

 chromosomes, so that when the two come together in this manner 

 the typical diploid, or somatic number, is restored in the combined 

 groups. It will be seen then, that if there were no reduction in the 

 course of the production of the gametes, each fertilisation would 

 result in the doubling of the chromosome number, a proceeding that 

 could not possibly go on. From this point on the division proceeds 

 as in typical mitosis. That is to say, the metaphase, anaphase and 

 telophase supervene, resulting in the formation of two cells. The 

 single ovum has thus been fertilised and has divided into two by 

 one continuous process, and this division is termed cleavage or 

 segmentation, and the resulting cells, the first cleavage cells or 

 blastomeres. It will be seen then, that there is no loss of chromo- 

 some identity in the above phenomena, and, particularly in those 

 cases where the chromosome groups do not mix, it is clear that each 

 daughter cell has its chromosome complement made up, half of 

 maternal and half of paternal chromosomes. The next division, or 

 second cleavage, takes place almost at once, and results in the for- 

 mation of the first four cleavage cells or blastomeres. In these, too, 

 it can sometimes be clearly seen that half the chromosomes are 

 descended from each parental germ cell, an important point in 

 considering the question of inheritance. 



The other method of fertilisation, while less common, is in 

 some respects more primitive and not so abbreviated. An amphi- 

 aster is formed as before, but slightly to one side, while the two 

 nuclei lose their membranes, and their chromatin granules, or 

 spiremes if they have been formed, mix up indistinguishably. This 

 results in the formation of one chromatin reticulum or spireme, 

 which may be termed the cleavage or fertilisation nucleus. The 

 amphiaster, sometimes not formed until this time, now takes up a 

 central position, and the spireme segments to form the diploid 

 number of chromosomes, which become arranged to form an 

 equatorial plate. Even in this type there is some evidence to show 

 that the paternal and maternal chromatin retains its individuality, 

 and the presumption is strong, that the cleavage cells receive an 

 equal share from both male and female parent. After the formation 

 of the amphiaster the blastomeres are formed as in ordinary division . 



In both types of fertilisation it is clear that the fertilised 

 ovum starts its career with a chromatin content derived in equal 

 parts from both parents. If, as we have reason to believe, the 

 chromosomes constitute the bearers of parental characters, then it 



