318 MESSES. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



their fate according to their position. If they He among the chromosomes inside the 

 spindle, they are carried with them to one or other pole and become included in 

 the daughter nuclei. If, however, they are left on the edge of the spindle, as 

 commonly happens with the larger vesicles, they remain outside the mitotic figure 

 in the cytoplasm and are not included in the nuclei of the daughter cells (text-fig. 15). 

 In this case they usually contract, and become small evenly-stained spheres, not 

 easily distinguishable from the larger yolk-granules, biit recognisable after the cell 

 division is completed, lying in the cytoplasni near the boundary between the two 

 cells (text-fig. 16). 



" In the second segmentation division, a similar process takes place, but it is usually 

 rather less pronounced ; the vesicles are, on the whole, smaller, and we doubt whether 

 complete chromosomes ever become vesicular." 



It is impossible to say from a study of the hybrid eggs whether the eliminated 

 chromosomes are derived from the male or female parent. A study of the effects of 

 hypertonic sea-water on the normally fertilized eggs of the two species has, however, 

 revealed considerable evidence in supjjort of the assumption that the vesicles in the 

 hybrid E. acittus % X E. esculentus $ are derived from the female parent [Gray (35)]. 

 This hypothesis receives considerable support from the fact that the elimination of 

 chromosomes hy means of " vesicle " formation is restncted to those hyhrids derived 

 from E. acutus eggs only — vesicles being only found in the crosses E. acutus $ 

 X E. esculentus $ and in E. acutus $ X E. miliaris ^ . 



2. E. acutus (aid E. miliaris Hybrids — • 



(a) E. acutus^ X E. miliaris $ (1911 and 1912). — In this cross a small number 

 of chromosomes show a varying tendency to swell and form vesicles in the early 

 stages of the first division ; where the tendency is pronounced this may cause 

 failure to divide in the metaphase, and they are carried to one or other pole. 

 Exceedingly small vesicles are sometimes eliminated, but this process is not frequent 

 and conspicuous, as it is in the cross E. acutus ? X E. esculentus $ . In the later 

 divisions the abnormal tendency is less marked, and by the third or fourth segmenta- 

 tion divisions all the chromosomes have regained their normal behaviour. In cases 

 where widely different numbers are counted at the two poles of one spindle in the 

 later segmentation divisions {cf I^pNCASTER and Gray, 1913, Plate 27 a and b), 

 it is possible that some chromosomes are still behaving abnormally ; but another 

 explanation, is that, owing to the irregularity of the equatorial plate, the two 

 halves of one chromosome are still seen in one daughter plate. 



Thus the cioss resembles tliat of E. acutus ^ X E. esculentus $ in that there is 

 a tendency for certain chromosomes to become visibly abnormal, although the 

 phenomenon is much less marked than in the latter hybrid. 



Another peculiarity in these eggs is in connection with the mitotic spindle. The 

 spindles in the hybrid are totally unlike those of E. ac^itus, and are not altogether 



