CLEAVAGE 223 



nearly always returns to a " resting" stage before its fusion 

 with the sperm nucleus, the latter may or may not do likewise 

 before the fusion of the two germ nuclei into the cleavage nu- 

 cleus; that the egg and sperm nuclei may or may not form 



o 



separate spiremes each with ^ chromosomes during the pro- 

 phase of the first cleavage figure of the zygote; and finally that 

 whatever the preliminary details may have been, the constant 

 and essential facts regarding this first cleavage figure are, (1) 

 that the chromosome group now consists of the full somatic 

 number of univalent elements, (2) that these are present in 

 pairs, and (3) that they have been derived equally from the 

 two parents. 



As cleavage begins the first important step is the longitudinal 

 division of each chromosome; the halves diverge during the 

 anaphase of the first mitosis, and into the nucleus of each 

 daughter cell there passes a precisely similar group of s chromo- 

 somes, paired as before and derived in equal numbers from each 

 of the two parents. In each succeeding mitosis the same thing 

 happens. So that in every cell of the blastula, and probably 

 even of the fully matured organism, the nucleus is composed of 

 substance derived in equal parts from the male and the female 

 parents (Fig. 106). In some forms (Copepods, Ascaris) the 

 two parental chromosome groups appear to remain fairly 

 distinct up to a late stage in cleavage (Fig. 107). And in 

 some cases of hybridization, when the chromosomes of the 

 two parents are easily distinguished by differences in size and 

 form (e.g., the hybrids of Fundulus and Menidia described by 

 Moenkhaus), such a process of equal distribution of the chro- 

 mosomes can be clearly followed into the blastula stage (Fig. 39). 

 It may fairly be assumed, therefore, that Huxley's comparison 

 of the body of an organism with a web, of which the warp comes 

 from one parent, the woof from another, has been justified by 

 the subsequently discovered facts of development. This idea 

 has been spoken of as the "autonomy of the male and female 

 chromosome groups." It follows from this that the parental 

 chromosome groups of the primordial germ cells of the new 



