II CLEAVAGE 45 



The membrane formed by these agents is apparently exactly 

 like that produced normally by the spermatozoon. We have 

 now to inquire what resemblance the division apparatus of 

 these parthenogenetie eggs bears to that which is made by the 

 sperm-centrosomes in a fertilized egg. 



The details have been revealed to us by the investigations 

 of Wilson into the cytology of magnesium chloride eggs, of 

 Hindle into that of those stimulated by the later butyric acid 

 method. 



The two accounts are in essential agreement as to the 

 origin of the division centres, but we shall follow Hindle in 

 the main, since the segmentation of these eggs at any rate 

 approaches the normal. 



The species used was Strongylocentrotus purpuratus. 



The following changes occur in the short interval (fifteen to 

 twenty minutes) between the removal from the butyric and 

 the immersion in the hypertonic solution. 



The membrane having been thrown off, the nucleolus 

 of the female nucleus, previously chromatic, loses its affinity 

 for basic dyes, and its definite shape ; it may fragment. 

 Meanwhile there appears a clear perinuclear zone of cytoplasm 

 from which radiations pass out in all directions. In the 

 hypertonic solution the nucleus enlarges, while the perinuclear 

 zone almost disappears, but, on removal to sea-water, reappears, 

 while the nucleus enlarges still more. 



Two cleavage asters are now developed with a spindle 

 between them ; each contains a centrosome. They are formed 

 by division of one aster and centrosome ; Wilson has shown 

 that the centrosome originates from, or at least at the 

 surface of, the nucleus. There are also in the cytoplasm 

 independent cytasters, each with its centrosome, which in 

 this case is not of nuclear origin. 



If exposure to the hypertonic solution is not too long, these 

 cytasters disappear and division takes place across the equator 

 of the * fertilization ' spindle, the female nucleus having first 

 broken up into chromosomes, which are present in the 

 reduced number (TI), a number which persists at least as far 

 as the blastula stage. 



