22 HARRIETT M. ALLYN. 



I. INTRODUCTION. 



The object of the experimentation described in this paper was 

 a partial analysis of the initiation of development in the egg of 

 the marine annelid, Chcstopterus pergamentacens. The experi- 

 ments were made and material collected at the Marine Biological 

 Laboratory in Wood's Hole during the summers of 1909, '10 

 and 'n 



Two methods of attacking the problem were employed, the 

 first, that of inducing artificial parthenogenesises-trie second, 

 that of combining fertilization and artificial parthenogenesis. 

 No investigator has yet succeeded in inducing normal develop- 

 ment by parthenogenesis in Chcztopterus, although varying degrees 

 of approximation to this result have been attained. A variety 

 of methods, which have proved more or less effective in other 

 forms, have therefore been employed, in the hope that such 

 experiments might throw light on certain of the factors concerned 

 in the initiation of development in this particular form. 



A. Normal Development. The egg differs from many eggs in 

 having a membrane present before fertilization. About ten 

 minutes after the entrance of the spermatozoan the first polar 

 body is extruded, and ten minutes later the second. The first 

 cleavage takes place forty minutes after fertilization, and is 

 preceded by the formation of a large polar lobe, the contents of 

 which passes into one of the two daughter cells. The first cleav- 

 age is thus unequal. The history of the cleavage is essentially 

 like that of other annelids. The blastula becomes ciliated, and 

 swims at six or seven hours. The larval form is a trochophore, 

 uniformly ciliated, with a long apical tuft of cilia at the anterior 

 end. Among any set of normal, fertilized eggs, there are very 

 likely to be found varying numbers of unsegmented, ciliated, 

 swimming larvae. Cleavage is thus seen to be unnecessary to a 

 certain amount of differentiation and development. This was 

 shown experimentally by F. R. Lillie (1902). These larvae do 

 not develop the typical form nor the apical tuft of cilia. 



B. Artificial Parthenogenesis. In artificial parthenogenesis the 

 development follows much the same course as in fertilization, 

 but the experiments enable us to resolve the development into a 

 separable series of steps. The first step is the formation of the 



