v KARYOMERES 131 



by each chromosome is a common occurrence in the cleavage divisions 

 of many animals. Instead of a single nucleus, we therefore find a mass 

 of small ones corresponding in number to the number of the chromosomes 

 (Fig. 61). As a rule this condition is temporary, the karyomeres generally 

 fusing later into a single nucleus. Certain abnormal conditions, e.g. high 

 temperature (Tobias, 1914), accentuate the tendency to karyomere 

 formation. 



FIG. 61. 



Formation of karyomeres in cleavage nuclei of various eggs. A. Chaetopterus pergamentaceus : ? nucleus 

 has completed its maturation divisions, and, in the form of a group of chromosomic vesicles is moving inwards 

 to meet the g nucleus (after Mead, J.M., 1895) ; B, C, resting and prophase nuclei from cleaving eggs of 

 Cyclops viridis, subjected to a high temperature (after Tobias, A.m. A., 1914); D, one nucleus of the 2-cell 

 stage of Polyphemus pediculus (after Kuhn, A.Z., 1908). 



A difficulty which has been urged against the view of the continuity 

 of the chromosomes is the supposed power of the nucleus to form chromo- 

 somes after amitotic division. As this matter has already been discussed 

 (p. 24) it need not be dealt with again. 



Very strong evidence in favour of the continuity of the chromosomes 

 has been obtained from the study of certain hybrids. Moenkhaus (1904) 

 crossed the Telostean fishes Fundulus heteroclitus and Menidia notata. 



