132 CYTOLOGY CHAP. 



The former has long (2-18 p), slender and generally straight chromosomes, 

 n being 18. The latter has about the same number of chromosomes, 

 which are short (i-oo /j) and generally curved. The eggs of each species 

 are fertilizable by the sperm of the other, and the hybrids so formed 

 develop for a certain period apparently normally, though in later 

 embryonic stages abnormalities make their appearance, and the eggs 

 seem incapable of hatching (see Loeb, 1912). The chromosome conditions 

 during the development of the hybrid embryo are especially interesting 

 (Fig. 62). After fertilization the male and female gamete nuclei fuse 

 in the resting condition. When the chromosomes appear for the first 





$m$& 



B 



FIG. 62. 



O 



A, anaphase of first cleavage mitosis in the egg of Fundulus heteroclitus B, similar figure from Menidia 

 notata ; C, D, anaphase of first (C) and later (D) cleavage mitosis of a hybrid (Menidia notata $ x Fundulus 

 heteroclitus <$) showing the two distinct types of chromosomes, separately grouped in C and mingled in D 

 (Moenkhaus, Amer. Journ. Anat., 1904). 



cleavage mitosis, however, it is found that the chromatin of the two 

 species has remained distinct, for the chromosomes appear in two groups, 

 one consisting of long chromosomes easily identifiable as derived from 

 the Fundulus parent, the other of short chromosomes derived from 

 Menidia. In telophase the two groups of chromosomes again become 

 indistinguishably merged into the resting nucleus, to reappear in the 

 same grouping at the next mitosis (2nd cleavage division). At the 3rd 

 cleavage the two types of chromosomes are still as sharply distinct 

 from one another, though they are no longer completely segregated into 

 two groups. By the 4th cleavage division the grouping is almost, and 

 in later cleavages quite, lost, the two types of chromosomes still, how- 

 ever, perfectly distinct being intermingled with each other. 



