MONOGRAPH OF THE EXISTING ORINOIDS. 591 



If the echinoid eggs are allowed to lie a long time in an alkaline solution 

 they become swollen when placed again in sea water, and fertilization, although 

 in a smaller percentage of cases, will take place without further treatment with 

 alkali. In the same way, if the genital capsules of Antedon have been previously 

 exposed to the hydroxyl (OH) ions the spermatozoa can fertilize echinoid eggs 

 in normal sea water. From this the inference is drawn that the hydroxyl (OH) 

 ions exercise an influence on the spermatozoa as well as on the eggs. 



Morphological investigation of the several developmental stages shows that 

 the stimulus for the development depends in the first instance on the penetration 

 of the spermatozoa into the egg. The raising of the yolk membrane is clearly 

 perceptible on the fertilized eggs, and examination also shows that the nucleus 

 of the Antedon sperm unites with that of the echinoid egg. 



Cell division proceeds in a manner entirely normal for the type represented 

 by the egg, and the time of the different cleavages coincides with that of the 

 eggs in the pure cultures. But since different eggs are fertilized at different times 

 after the mixture of the eggs and sperm, and therefore commence to develop at 

 slightly different periods, there are found side by side in the same vessel eggs 

 in different cleavage stages. 



The blastula stage also can not be differentiated from the blastula of the 

 pure echinoid culture. The blastulse give rise to cilia at the appointed time and 

 swim nimbly about the vessel. 



The further development depends upon the echinoid species from which the 

 eggs are derived. 



Of the Sphcerechinus X Antedon embryos only a small part reached the gas- 

 trula stage, and the commencement of skeletal formation was never detected. 



The Paracentrotus X Antedon hybrids for the most part reached the gastrula 

 stage, and individuals among them reached the pluteus stage. 



The largest number of plutei were from the Echinus X Antedon cross. 



During the course of development a halting of the morphogenetic processes 

 is often at times to be observed first during the blastula stage from the com- 

 mencement of mesenchyme formation, and later after the termination of gastru- 

 lation and before the commencement of the formation of the skeleton. This 

 halting of the developmental processes often lasts for several hours and, especially 

 in the Sphterechinus X Antedon cross. Godlewski often noted an interruption of 

 development lasting for two days. In each of such periods a part of the embryos 

 sink to the bottom, while the remainder develop further. 



The percentage of the eggs which reach the pluteus stage is as a rule very 

 small, and depends very much on the individuality of the material. At the most, 

 5 per cent of the Echinus X Antedon hybrids develop to the pluteus stage, and 

 usually the proportion was very much less. Often there occurred cases where 

 no eggs developed beyond the stage in which the gastrula shows the commence- 

 ment of the formation of the three-rayed skeleton. 



During the whole development the characters are exclusively those of the 

 mother animal, the only differences between these hybrids and normal echinoids 



