THE REALISATION OF THE NUCLEAR FACTORS 



77 



echinus granularis with sperm of either Psammechinus or Para- 

 centrotus. The plutei of these forms (Fig. 29) show marked 

 differences. Those of Spkaerechinus are of a plumper shape; 

 the relative lengths of the arms are different, and the structure 

 of the calcareous rods is more complicated. One of the latter, 

 the anal rod, consists of 3-5 parallel rods with cross connections. 

 Another, the apical rod, has three prongs at its top. In Para- 

 centrotus, the anal rod is single, and the apical rod club-shaped, 

 but not branched (in Psammechinus the latter shows an 

 incipient branching; see below p. 83). Now it was found that 

 the hybrid larvae were intermediate, both in shape and in 



Fig. 29. Pluteus larvae of (a) Sphaerechinus granularis, (h) Psamm- 

 echinus micro tuberculatus, and (c) the hybrid Sphaerech. gran. X 

 Psammech. microtuberc. After Boveri. 



structure of the skeleton. Their anal rods consisted of 2-3 rods, 

 without cross connections; their apical rods were club-shaped, 

 but also showed a tendency to branch (Boveri, 1889-95). It is 

 a remarkable fact that, apart from intermediate larvae, other 

 larvae are sometimes found with a preponderance of paternal 

 characters ("patrocline" larvae). Conditions in the culture 

 seem to play a role here ; in particular the temperature appears 

 to influence the structure of the calcareous skeleton. 



In this case, the influence of the paternal chromosomes on 



