298 LECTUBES ON BIOLOGY 



Indeed, Boveri and other investigators succeeded in obtaining 

 from such non-nucleated the fertilized fragments of sea-urchin 

 ova fully developed Pluteus-larvse which were distinguished from 

 the normal larvae only by their smaller size, corresponding to the 

 smaller quantity of food material. As in the parthenogenesis of 

 the egg-nuclei, so in cases of fertilization of non-nucleated egg- 

 fragments, which we will describe with Kawitz as ephebogenesis, 

 the sperm-nucleus alone suffices to produce a complete organism. 

 These experiments corroborate, therefore, our assumption that 

 only want of protoplasm prevents the spermatozoon from pro- 

 ceeding independently to division and development. 



But Boveri carried his experiments still further, and was able 

 to demonstrate that animals bred from non-nucleated fragments 

 possessed only purely paternal characters, and that therefore 

 only the nucleus transmits heritable characters. He proved 

 this by the method of crossing. In the two species of sea- 

 urchins, Echinus and Sphcer echinus, the Pluteus-larvse differ 

 greatly in the shape of their body and the structure of the lime- 

 skeleton. The larva of Echinus is slender, that of Sphserechinus 

 plump. Boveri convinced himself first of all that by a normal 

 process of bastardization, i.e., impregnation of complete ova, or 

 at any rate nucleated egg-fragments, of Sphserechinus with the 

 spermatozoa of Echinus he obtained mixed larvse which ex- 

 hibited paternal as well as maternal characteristics, and held 

 the mean between the two extreme types. B.ut if spermatozoa 

 of Echinus invaded non-nucleated fragments of Sphserechinus, 

 the result were larvse of the pure Echinus type, i.e., larvse pos- 

 sessing only paternal characteristics. Unfortunately Boveri was 

 unable, owing to technical difficulties, to carry out these impreg- 

 nating experiments with isolated, non-nucleated egg-fragments, 

 so that his observations are open to objections. But as the 

 nuclei of the larvse of the pure Echinus type were considerably 

 smaller than the typical bastard-larvse of the same size a pheno- 

 menon that can only be understood on the assumption that they 

 received only half the normal chromatin quantity, i.e., only one 

 of the two paternal nuclei it may nevertheless be accepted as 

 proved that they originated from fertilized non-nucleated egg 

 fragments. 



However necessary the fertilizing act may be for the begin- 



