Studies in Heredity. 



243 



tendency to cytolysis, which inhibits altogether the development of most 

 eggs of Echinus when treated with the sperm of Uchinocardium. 



The next day the hybrid blastula becomes a gastrula by the appearance 

 of the invagination which gives rise to the archenteron. This invagination 

 is of very small size compared to the corresponding invagination in a normal 

 gastrula — it is, so to speak, pushed to one side by the abnormal multitude 

 of mesenchyme cells (fig. 3). On the third or fourth day the larva reaches 

 the stage when the normal larva is denominated a "prism-larva" — the 



Arch. 



Fig. 3. — Gastrula produced by fertilising the egg of Echinus esculentus with the sperm of 

 Echinocardiura cordatv/m. Arch, archenteric invagination. 



stage, that is to say, when the archenteron has become differentiated into 

 oesophagus, stomach and intestine, and when the mouth and stomodasum 

 are making their appearance as a new invagination. The hybrid, however, 

 has retained the spherical form of the egg ; it is filled with a great mass 

 of mesenchyme, and the small and feebly developed alimentary canal lies 

 pushed to one side (fig. 4). 



A few of these hybrids lived to the age of five or six days, and, whilst the 

 form remained mainly spherical, two short feeble rudiments of the postoral 

 arms were produced. 



Loeb,* in describing experiments with the eggs of Asterias, makes the 

 statement that these eggs require to lie in sea-water for some time in order 

 to ripen : and that after they have lain for a day or so in sea- water without 

 being fertilised they die and undergo granular degeneration. It occurred 

 * ' Die Chemische Entwicklung des Tierischen Eies,' Jena, 1910. 



