Habits and Early Development of Lincrges mercurius. 167 



these nuclei are no nearer the periphery than in the case of many other eggs 

 (mollusks, annehds, etc.) in which unilateral constriction does not occur. 

 Again, the heaping up of the protoplasm at the animal pole is by no means 

 as great as in many other animals, such as the annelids and mollusks, in 

 which unilateral constriction does not take place. Therefore, the cause of 

 the peculiar form of cell-division found in the early cleavage of the egg of 

 Linerges and other coelenterates is not satisfactorily explained by any of 

 these hypotheses. 



In most animals the cleavage of the egg begins at the animal pole, 

 where the protoplasm is most abundant, and the cleavage- furrow then 

 extends around the egg and gradually cuts in from all sides toward the 

 center. In such cases the center of the egg is quite as firm as is the peri- 

 phery. C)n the other hand, the center of the egg is less firm in some 

 coelenterates (perhaps in all) than is the periphery, and it seems probable 

 that the unilateral constriction in the division of these eggs is partly due 

 to this condition ; for after the furrow has cut through the peripheral lavers 

 to the more fluid central area it would then progress rapidly toward the 

 vegetal pole. This type of cleavage prevails during the first, second, and 

 third cleavages, in fact as long as the more fluid central area is present, but 

 with the disappearance of this area in the formation of the cleavage cavity 

 and its contents, this type of cleavage disappears, and the ordinary type 

 thereafter occurs. Taken in conjunction with the heaping up of the peri- 

 pheral layer at the animal pole, this is, I believe, an explanation of the uni- 

 lateral type of constriction of the eggs of coelenterates. 



In conclusion, I desire to express my thanks to the Carnegie Institution 

 of Washington and to Dr. Alfred G. Mayer, Director of the Tortugas Labor- 

 atory, for the generous assistance given me while a guest of the Laboratory. 



