78 SCIENCE PROGRESS 



a monocaryon, and development will only continue if the 

 egg has been treated with hypertonic sea-water (the second 

 factor). 



In fertilisation of Sphaerechinus egg by Chaetopterus sperm 

 the two nuclei unite, but before mitosis the paternal chromatin 

 is extruded ; here, again, treatment with hypertonic sea-water 

 is necessary for development. 



Cases are also known where the action of a second factor is 

 not necessary, as, for example, an Echinus egg fertilised by 

 sperm from Andouinia ; where, in spite of the great difference 

 in the parents, the two nuclei remain as an amphicaryon 

 for a long time. 



An intermediate condition is found in the &gg of Strongylo- 

 centrotus fertilised by Sphaerechinus spermatozoa : the paternal 

 chromatin is extruded at the metaphase of division, but, as 

 mitosis commences with an amphicaryon nucleus, no second 

 agent is required. Finally, Baltzer showed that when a 

 Stronglyocentrotus female is fertilised by Arbacia sperm 

 development proceeds normally until the amphicaryon nucleus 

 becomes monocaryon by extrusion of chromatin. This stage 

 marks a crisis in development, which, in many cases, leads to 

 the death of the embryo. 



It will be obvious from the foregoing that Herlant's con- 

 clusions are exceedingly suggestive, and further show that the 

 detailed cytological study of artificial parthenogenetic eggs 

 will yield results of wide interest to experimental biologists. 



