EARLY STAGES OE SEGMENTATION OE ECillNES HYBEIOS. 483 
Cytological Observations on the Early Stages 
of Segmentation of Echinus Hybrids. 
By 
L. Doncaster, M.A., 
Fellow of King's College ; and 
.r. Gray, B.A., 
Scdiolar of King's College, Cambridge. 
With Plates 28 and 2!>. 
Introduction. 
The material on wliicli the greater part of the observations 
here described was made was handed over to us by ]\[essrs. 
Shearer, De Morgan and Fuchs, and was collected by them 
while carrying out experiments on the characters of the plutei 
in hybrids between the British species of F chin ns. A pre- 
liminary account of their results has been published,' and 
their fuller paper appears concurrently with this. In view of 
the fact that in some of the hybrids the plutei, whichever way 
the cross was made, were essentially of the maternal type, it 
was clearly of interest to determine whether an elimination 
of paternal chromosomes takes place comparable with that 
described by Jfaltzer,- who has found that iii the early stages 
of hybrids between different Ecliinoid genera, paternal 
chromosomes are eliminated in those cases in which the plutei 
are of the maternal type. The species used in the present 
exjieriments were Echinus esculentus, E. acutus, and 
E. miliaris, the three species of Echinus found at Plymouth, 
where the experiments were made. The maten'al obtained 
‘ ‘ Journ. Marine Biol. Soc.,’ ix, Itdl, p. 121. 
“ ‘ Arch. f. Zellforscli.,’ v, ItdU, 497. 
