316 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



purely maternal inheritance in certain hybrids cannot be explained by means of 

 Kupelwieser's work. 



In 1908 Baltzek (6) published his extensive account of tlie early segmentation 

 stages of cross-fertilized eggs, and also gave a description of the larvse to which such 

 eggs give rise. His most definite results were in respect to the reciprocal hybrids 

 of Sphcerechinus granularis and Strongylocentrotus lividus. In accordance with the 

 results of Vernon, Herbst, and others, he found that the cross Strongylocentrotus ? 

 X Sphcerechinus $ was very difficult to obtain, but those plutei which he obtained 

 were of an intermediate type, and the nuclei of the hybrid larvse were of such a size 

 as would be expected from a nucleus which contained all the chromosomes from both 

 male and female pronuclei. In the reverse cross, however, viz., in Strongylocentrotus $ 

 X SphcBrechiitus $ , Baltzer obtained only larvae of the maternal type, while an 

 investigation of the first segmentation divisions of the fertilized egg showed that a 

 considerable number of chromosomes failed to be included in the daughter nuclei 

 and eventually degenerated in the cytoplasm. Baltzer brings forward strong, 

 though not conclusive, evidence that these omitted chromosomes are derived from the 

 male parent. From the results of his work he makes a most important deduction, 

 namely, that the purely maternal skeleton of certain hybrids is dependent upon the 

 omission from the nucleus of those paternal chromosomes which are generally connected 

 with skeleton production. 



Description. 



The cytology of the various hybrids discussed in this paper has been fully investi- 

 gated by Doncaster and Gray (24). The following is a brief summary of their 

 results. It should be noticed that the material for the cytological work was the 

 same as that from which the hybrid plutei were reared. 



1. E. esculentus and E. acutus Hybrids* — 



" In the cross E. escidentus ^ X E. acutus $ , the mitotic figures of the segmenting 

 egg are perfectly normal, and do not difier recognisably fiom those of the pure 

 species. In the converse cross, E. acutus % X E. esculentus $ , however, a striking 

 abnormality is constantly present in all the eggs examined. Until immediately 

 after the dissolution of the nuclear membrane in the first segmentation division 

 the behaviour is regular, and 38 normal chromosomes can be counted. As the 

 spindle is formed the chromosomes become scattered upon it irregularly, and 

 gradually become collected in the equatorial plate. During this process it is then 



* It is somewhat unfortunate that it should be impossible to find any diagnostic features in the larvse 

 of E. acufm and E. car.ulcntm, by which it might be possible to determine whether the loss of chromosomes 

 in the cross E. acutus ? x £. esculentus (^ had any connection with the inheritance of larval characters. 

 As, however, the cytology of this cross throws some light upon the nature of " elimination " in general, 

 we give a short extract from the observations of DoNCASTER and Gray. 



