ON THE EXPEEI MENTAL HYBEIDIZATION OF ECHINOIDS. 321 



to be correlated with the omission from the egg of corresponding chromosomes derived 

 from the male parent, reference must be made to the work of Tennent, who 

 summarises the present position as follows : — 



1 . Retention of all chromosomes, and dominance of one species over another with 

 respect to skeletal characters. 



2. Elimination of part of the chromatin, and dominance of one species over another 

 with respect to skeletal characters. 



3. Elimination of part of the chromatin, and intermediate skeletal characters. 



4. Elimination of part of both paternal and maternal chromatin, and inhibition of 

 development. 



Tennent holds that the first three, at least, may all occur in a given lot of eo-o-s 

 and depend partly upon chance, while the second and third cases may indicate an 

 incompatibility between the chromosomes and the cytoplasm of the egg ; the fourth 

 case shows also an incompatibility between the chromosomes themselves. He does 

 not commit himself either in favour of or against Baltzer's suggestion. 



The case of elimination of botli paternal and maternal chromatin refers to the 

 crosses made between the genera Arbacia and Toxopneustes, although what exactly 

 is meant by an " incompatibility between the chromosomes themselves " is not quite 

 clear. As pointed out elsewhere (Gray, 35), the elimination of both male and female 

 chromosomes is distinctly adverse to Baltzer's idea that the male chromosomes are 

 eliminated simply because they are " out of tune " with the cytoplasm of the egg. 

 GoDLEWSKi (30 and 31) has shown that in the hybrid Echinus % X Antedon $ the 

 larvae are apparently of the maternal type, and yet there is no elimination of 

 chromosomes in the early segmentation divisions. Baltzer has confirmed these 

 results, and explains them by suggesting that tlie efiect of the foreign cytoplasm 

 upon the paternal chromosomes is to cause them to become latent, and so they 

 do not impress any paternal characteristics upon the hybrid larvae. 



Such a suggestion, as shown above, nullifies his explanation of the behaviour of 

 the chromatin in the cross Strongylocentrotus $ X SjylicerccJiinus $ . 



In a recent paper, Kupelwieser (49) supports Baltzer's suggestion that the 

 development of hybrid larvae is intimately connected with the phenomena of the 

 early segmentation divisions of the egg. He holds that the asymmetrical larvae which 

 are obtained by fertilizing the egg oi Echinus by the sperm of the Polychaet Auduinia 

 are to be explained by the abnormalities induced in the female chromatin by the 

 mechanical presence of the functionless male chromatin. In short, he concludes : 

 " Uurch die in den ersten Furchungen vorkommenden Storungen der Mitose werden 

 die Defekte an den Pluteen jedenfalls geniigend erklart. Das fremde Chromatin 

 wird diese Storungen wahrscheinlich rein mechanisch durch unregelmassiger 

 Inanspruchnahme der Zugfasern hervorrufen " (pp. 380, 381). 



VOL. cciv. — B. 2 t 



