THE REALISATION OF THE NUCLEAR FACTORS 



83 



The enucleated eggs were then fertilised. In this way he obtain- 

 ed hybrid merogones between Psammechinus microtuberculatus 

 and Paracentrotus lividus. These developed well, and produced 

 harmoniously built plutei. It is true that the plutei of the parent 

 species are very similar to each other, but after a very minute 

 investigation, Horstadius was able to demonstrate, by means 

 of slight differences found in the shape of the calcareous rods, 

 that the sperm nucleus had a clear-cut influence on these 

 organs. The tips of the apical rods are slightly more irregular 



Fig. 30. Apical rods of the skeleton of the pluteus larva of (a) 

 Psammechinus microtuberculatus, (b) Paracentrotus lividus, and 

 (c) the hybrid merogone Parac. liv. (cytoplasm) X Psammech. 

 micro tub. (nucleus). In the latter, the shape of the apical rods is 

 more similair to that of Psammechinus. After Horstadius. 



in Psammechinus than in Paracentrotus j, also they show a 

 slight tendency to branch. In this respect, the hybrid merogones 

 behaved in the same way as the species that supplied the sperm 

 nucleus (Fig. 30). Evidently this character is governed by the 

 nuclear factors. Some further information on their mode of 

 operation may be obtained from the combination of this result 

 with observations by von Ubisch (1937) on the way in which 

 the calcareous skeleton of the pluteus develops. It is built by 

 mesenchyme cells of the gastrula, which are grouped to the 

 left and right of the archenteron, and fuse into a so-called 

 syncytium. The material of the rods is then secreted within 

 this syncytium (Fig. 32). The viscosity of the cytoplasm of 

 these cells appears to be an important factor in the final 



