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. 
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 
