ON THE DEVELOPMENT OF SAGITTA. 351 



On the Development of Sagitta; with Notes on 

 the Anatomy of the Adult. 



By 



I^. Doiicastor, 



King's College, Cambridge. 



With Plates 19—21, 



I. Historical. 



The development of the Chsetognatha was first investigated 

 by Gegeiibaur, but tlie earliest full account was given by 

 Kowalevsky in 1871 (1). He described the gastrulatiou, the 

 formation of the archenteric pouches, and the subsequent 

 stages up to the hatching of the young Sagitta, Biitschli 

 studied another species taken off the coast of Norway in 1873 

 (2), and found considerable difference between this species 

 and that wliich Kowalevsky had described. The most 

 important new facts which he added were, firstly, the forma- 

 tion of separate anterior cavities of the ccelom, separated off in 

 the earlier stages of the embryo from the archenteric pouches ; 

 and secondly, the very early separation of the cells which 

 remain unaltered during the whole embryonic development, 

 and which ultimately give rise to the male and female genera- 

 tive organs. 



In the species studied, which unfortunately was not iden- 

 tifieJ, the tissues of the embryo had a much more epithelial 



