VI ECHINODERM HYBRIDS 157 



developed as far as gastrulation and in a few cases even reached the 

 pluteus stage. They exhibited solely the characters of the female parent 

 {Echinus). Godlewski even managed to fertilize eggs of Echinus from 

 which the nuclei had been removed (by shaking) with Antedon sperm. 

 These all developed to embryos of pure Echinus type. None of them, 

 however, passed the gastrula stage. From these experiments Godlewski 

 argues that the egg cytoplasm and not the nucleus is the determining 

 factor in early development. 



Boveri's classical experiments on the cross Sphaerechiniis 2 x Echinus S 

 led to exactly the opposite conclusion (1896). These larvae are normally 

 intermediate between the two parents. By violently shaking the eggs 

 before fertilization, Boveri succeeded in breaking many of them into 

 two or more fragments, of which of course only one contains a nucleus. 

 On adding Echinus sperm to a mass of these broken-up Sphaerechinus 

 eggs a culture was obtained which consisted of a mixture of — (i) 

 normal larvae, intermediate between the two parents ; (2) dwarfs, also 

 intermediate ; and (3) dwarfs, resembling the male parent exclusively. 

 Having pre\'iously, by direct observation, ascertained the power of 

 non-nucleated egg fragments to be fertiUzed and to develop normally, 

 Boveri interpreted these three classes as having originated as follows : 

 (i) from unfragmented eggs, (2) from egg fragments containing a nucleus, 

 (3) from egg fragments containing no nucleus — all of course fertilized 

 by Echinus sperm. From this result Boveri draws the conclusion that 

 the nucleus is the sole bearer of hereditary factors. Taking, however, a 

 general view of the results of Echinoderm crosses, it undoubtedly is the 

 case that the hybrids in the larval stages do tend to resemble the female 

 parent more than the male, though the tendency is by no means a 

 universal rule, and though the prepotency of one or other parent is 

 strongly influenced in some way or other by factors of the external 

 environment. 



One cause of the frequent prepotency of the female parent in 

 Echinoderm crosses has been discovered by Baltzer (1910) in a work 

 which is of the greatest importance in interpreting the results of crosses 

 between distantly related forms. To take the case of two Echinoderm 

 genera, Strongylocentrotus and Sphaerechinus, the cross Sphaerechinus ? 

 X StrongylocentroUis S gives embryos which develop regularly and with- 

 out pathological phenomena into plutei, which as regards the skeleton 

 are intermediate between the two parent species. Cultures from the 

 reciprocal cross, however, Strongylocentrotus ? x Sphaerechinus S run a 

 different course. When the hybrid embryos reach the blastula stage, 

 instead of turning into a hollow sphere, the cavity (blastocoele) becomes 

 filled with masses of degenerating cells and nuclei, and the larvae, which 

 should be transparent, become in consequence opaque and the great 



