Ill DIFFERENTIATION 95 



teristic skeleton are developed. The starfish larva is of course 

 the Bipinnaria, without skeleton. The problem was taken up 

 next by Godlewski. who used the Crinoid Antedon as the male 

 parent, the Sea-urchins Sphaerechinus, Echinus > and Strongy- 

 locentrotus as the female. The sexual elements must first be 

 treated with hyperalkaline sea-water. 



Fertilization is perfectly normal with production of a 

 vitelline membrane, rotation of the sperm-head, union of the 

 male and female pronuclei, and so forth. Cleavage is of the 

 Echinoid type with formation of micromeres at the fourth 

 division ; no such micromeres occur in Antedon, where the 

 fourth cleavage is meridional in both hemispheres. Primary 

 mesenchyme (absent in Antedon) is formed, gastrulation 

 follows, and then a typical pluteus with the characteristic 

 skeleton is developed. The larva of Antedon has no skeleton. 

 The Antedon chromosomes persist, and the nuclei of the 

 hybrids are intermediate in size between those of the parent 

 forms. Baltzer, who has repeated the experiment, confirms 

 this as well as all other details. 



Enucleate egg-fragments of Echinus were also fertilized by 

 Godlewski with Antedon sperm. The details of fertilization 

 were normal, but segmentation was irregular, and after gastru- 

 lation development ceased. Primary mesenchyme was differen- 

 tiated and the archenteron inclined to the oral side, both 

 Echinoid characters. 



In spite, therefore, of the persistence of the male chromo- 

 somes the larva is of the pure maternal type. 



The Sea-urchin egg may also be fertilized by the spermato- 

 zoa of Molluscs and Worms. 



Loeb, employing the Mollusc Chlorostoma, found that a 

 membrane was extruded, that cleavage was normal in form 

 and rate, and that normal plutei were developed. The cytology 

 was not investigated. 



A more extensive series of experiments is due toKupelwieser. 

 If the egg of Strong ylocentrotus or Echinus be fertilized with 

 the sperm of the Mussel Myiilus, no membrane is formed and 

 polyspermy is frequent. The sperm-nucleus having rotated, 

 preceded by its sphere, moves towards the egg-nucleus. 



