NatTuRAL DEATH AND FERTILIZATION 729 
avery large plainly visible nucleus.’ The process of matura- 
tion consists morphologically in this, that the nucleus 
becomes invisible and the polar bodies are thrown out. 
This process is completed within one or two hours after 
the eggs are removed from the ovaries and placed in sea- 
water. Only when maturation is complete is it possible to 
cause the egg to develop through the addition of sperm or 
through the physical and chemical agencies that have been 
described by me, Delage, Mathews, and Greeley. 
Il. THE NATURAL DEATH OF THE MATURE UNFERTILIZED 
STARFISH EGGS 
The living eggs of Asterias are light yellow in color and 
homogeneous. They retain this appearance during the pro- 
cess of maturation as long as they are alive. They retain 
this appearance also when they are made to develop through 
the entrance of a spermatozoon or through the proper 
chemical or physical means. 
If, however, the mature eggs are not fertilized or do not 
develop, they die in the course of four to twelve hours, and 
this process of dying is accompanied by a characteristic 
change in the color of the egg. The egg becomes at first 
opaque, then almost black, and the homogeneous structure of 
the protoplasm becomes granular. If such a culture of 
unfertilized eggs is examined under the microscope after 
twenty-four hours, two kinds of eggs are found, first, the 
just-described dark, dead eggs which are mature, and 
secondly, living, normally colored, but immature, eggs. For 
usually not all the eggs that are removed from the ovaries of 
a starfish mature at once; many mature very late, others not 
at all. It is readily seen that the immature eggs remain 
1The recent beautiful experiments of Delage have shown that, besides these 
visible changes in the nucleus, chemical, but morphologically invisible, changes also 
occur in the protoplasm. DELaGE, ‘“ Etudes expérimentales sur la maturation 
cytoplasmique et sur la parthénogendse artificielle chez les Hchinodermes,” Arch. de 
zoologie expériment., Vol. IX (1901). 
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