336 bulletin: museum of comparative zoology. 



figures of chains by Pouchet (1894), Karsten (1907), and Kofoid 

 (1908, fig. 10) where the antero-posterior gradation is very marked. 

 The chains of Ceratium are in active locomotion during chain forma- 

 tion. It is quite possible that the direction of locomotion may in 

 some way influence the distribution of the plasma of the parent cell 

 to the daughters in such a way that the posterior member receives 

 the greater part. The resistance of the medium to locomotion in an 

 anterior direction would tend to have this effect upon the plastic 

 living substance. 



Far more significant than this antero-posterior dissimilarity in the 

 members of the chain is the progressive reduction in size towards 

 both of the distal ends of the chain. This is evidently due to a domi- 

 nating factor which overrides the tendency for the posterior member 

 to be larger than the anterior above referred to. It is practically 

 impossible with the data at hand to unravel the sequence of cell 

 divisions which gave rise to this chain. Two possible causes suggest 

 themselves as operative, first, unequal cell division, with the smaller 

 member in each case the distal one, but the inequality less in the pos- 

 terior region than anteriorly; second, more rapid cell division in the 

 distal regions. The volumetric relations of the members of the chain 

 are such that the first to the fourth and eighth to eleventh individuals 

 may be one or even two generations older than members five, six, and 

 seven. Assuming that the eleven members constitute the complete 

 chain, and this seems probable from the condition of the plasma 

 at the ends. It is hardly possible to devise a scheme of descent with- 

 out bringing in both unequal division and inequality in the rate of 

 division. This inequality in the results of the process of schizogony 

 has an important bearing on the interpretation to be put upon the 

 chain in question, and in differentiating it from previously known 

 forms of schizogony in the Dinoflagellata. 



Schizogony in the Dinoflagellata is of several kinds each of which 

 bears some relation to the presence or absence of exoskeleton or theca 

 and to the part which this, when present, plays in the process. The 

 extent to which protoplasmic continuity is retained after nuclear 

 division determines the possibility of chain formation. The following 

 table exhibits a classification of types of schizogony and the place 

 which the type here presented occupies in the scheme. 



Body naked 



Schizonts parted at once on fission. Most Gymnodinidae. 

 Schizonts retaining temporary association in cyst. Gymnodi- 

 nium lunula. 



