260 



INTERNAL FACTORS 



IV. i 



A mass culture of egg-fragments of Sphaerechinus containing 

 presumably whole eggs, and nucleate and enucleate fragments 

 was then made, and this was fertilized with the sperm of Echinus. 

 Amongst the larvae developed in such a culture Boveri found 

 a small number (twelve) of dwarf individuals which possessed the 

 paternal (Echinus) characters alone (Fig. 159 b], and he suggested 

 that these had come from enucleate fragments of eggs. He was 

 further strength 3ned in. this opinion by the fact that the nuclei 

 were small, which he attributed to their containing only one-half 

 the normal number of chromosomes. 



This conclusion has been adversely criticized by Seeliger. 

 Seeliger, working at Trieste and subsequently at Naples, has 



FIG. 158. Pluteus of the cross Sphaerechinus granularis x Echinus 

 microtiiberculatus a*, from in front and from the side. (After Boveri. 

 1896.) 



pointed out that the differences between the Plutei of these two 

 sea-urchins are not as great as Boveri asserted them to be the 

 extremity of the apical arm in Echinus is, for instance, not always 

 club-shaped and that therefore it cannot be so confidently asserted 

 that the hybrid is intermediate. Secondly, he asserts that in an 

 ordinary hybrid culture (whole eggs of Sphaerechinus fertilized by 

 sperm of Echinus) there are to be found together with variable 

 larvae of more or less an intermediate type, individuals with purely 

 paternal characters, though the pure Sphaerechinus type never 

 occurs, and that the nuclei are variable in size. This contention 

 has been upheld by Morgan, and, after a good deal of controversy, 

 has finally been admitted by Boveri himself. No conclusion can 

 therefore be drawn from the original experiment. 



