428 WESLEY R. COE, 



The Living Egg. 



The eggs of Cerehrafulus leidyi^) can easily be obtained in 

 abundance during the month of August, and develop readily for a 

 period of six or eight days. The species is very common along the 

 southern shores of New England, and is usually found at about half- 

 tide in sheltered localities where there is a mixture of sand and mud. 



The egg as it leaves the animal is somewhat irregular in shape, 

 but soon acquires a nearly spherical form, and measures about 

 0,085 mm in diameter. The large germinal vesicle lies somewhat 

 eccentrically near the middle of the egg. In about six minutes there 

 is seen a pair of small indentations of the nuclear membrane. These 

 appear on the side of the nucleus which is nearest the periphery of 

 the cell, much as Griffin (7) has described for Thalassema. So that 

 at this early stage, and, perhaps indeed, even before the eggs leave 

 the body of the parent, the cell possesses a marked polarity, and the 

 plane of cleavage has already been more or less precisely determined. 

 For here, as in many other animals, the plane of cleavage passes 

 nearly through the point where the polar bodies are given off; this 

 point is apparently determined within certain bounds by the side of 

 the nucleus on which the pair of asters are formed, and their position 

 is, in turn, dependent on the position of the eccentrically placed 

 nucleus in the egg. 



In each of the two indentations of the nuclear membrane above 

 described, a delicate aster gradually becomes visible. These indent- 

 ations increase in size with the growth of the asters, and the outline 

 of the nuclear membrane becomes much fainter in their immediate 

 vicinity, and gradually disappears. Up to this point the two asters 

 have developed apparently independently of each other, but now the 

 radiations of the two begin to meet and to form a central spindle 

 between them. Meanwhile the remaining portion of the nuclear mem- 

 brane disappears (although far distant from the aster-fibers) and the 

 nuclear substance merges into the cytoplasm. The nucleolus becomes 

 vacuolated and disappears. The amphiaster now rotates until it comes 

 to lie in a radius of the egg, nearly at right angles to its former 

 position , with one pole on the periphery. The surface of the egg 

 bends inward slightly at the point of contact with the periphery ^\ 



1) Verkill (27). 



2) This shallow depression appears to be equally conspicuous in 

 eggs which have been fertilized, and in those which have not. 



