XXI 



IS DEVELOPMENT OF THE SEA-URCHIN EGG POSSIBLE 



WITHOUT MEMBRANE FORMATION OR WITHOUT 



THE SECOND (CORRECTIVE) FACTOR? 



1. It has been known for a long time that if sea-urchin eggs 

 lie for some time in sea-water they may begin to segment. I 

 have recently investigated this phenomenon in the eggs of 

 S. purpuratus, and find that such a segmentation is always 

 preceded by membrane formation. 1 



If the unfertilized eggs of S. purpuratus are kept at a rela- 

 tively low temperature, one notices that after twenty-four to 

 forty-eight hours, in a part of the eggs of certain females, a 

 membrane formation appears of that type which is common in 

 the eggs of Arbacia after an acid treatment. Around the eggs 

 a gelatinous layer is formed. If such eggs are kept at a low 

 temperature and with sufficient oxygen supply, they begin to 

 segment and this segmentation may proceed to the eight- or 

 sixteen-cell stage almost regularly (Figs. 66-72). Only those 

 eggs segment that form a membrane. The eggs of certain 

 females only show this tendency and the number of purpuratus 

 females whose eggs form membranes spontaneously is very 

 small. It can be shown that the eggs which form membranes 

 spontaneously behave in the same way as the eggs in which 

 the membrane formation is called forth by butyric-acid treat- 

 ment. At a high temperature they disintegrate at the time of 

 the first segmentation; at a low temperature they can segment. 

 Moreover, the eggs which form membranes spontaneously can 

 be caused to develop into larvae by a short treatment with 

 hypertonic sea-water. 



The question arises, What causes this spontaneous membrane 



1 Loeb, Archiv f. Entwicklungsmechanik, XXXVI, 626, 1913. 



219 



