352 L. DONCASTER. 



cliaractei" tlian those observed by other workers, and the 

 structure of tlic embryo is therefore almost diagrammatic in 

 its simplicity; the development after hatching was, however 

 not followed. 



The most complete and accurate account of the embryology 

 of the group which has yet appeared was published by O. 

 Hertwig ill 1880 (3). Besides making a careful investigation 

 of the structure of the adult in several species, he gives a full 

 description of the development, from the first cleavage of the 

 egg np to the tenth day after hatching. He described for 

 the first time the remarkable cleavage, and pointed out that 

 the cells which give rise to the sexual organs are at first two, 

 and later four, and not two or four groups of cells, as was 

 stated by Biitschli. He gives a fuller and more accurate 

 account than his predecessors of the formation of the mouth 

 and alimentary canal, and suggests, without asserting 

 defiuitelj^ that the longitudinal septum in the tail region of 

 the adult is formed by a continuation of the alimentary canal 

 into the tail, and not by the prolongation of the splanchnic 

 mesoderm, as Kowalevsky believed. 



Hertwig, however, failed to confirm Biitschli's observation 

 on the origin of the anterior pair of coelomic cavities. After 

 the young Sagitta has hatched he describes the formation of 

 the body-cavities, which have lieen obliterated in the embr}'©, 

 the migration of the genital cells from the splanclinic meso- 

 derm to the body-wall, and mentions that the transverse septa 

 across the body appear also at the same time. He also men- 

 tions the formation of a lumen in the intestine, the ciliation 

 of its walls, and refers to the origin of the anus. The origin 

 of the muscles and nervous system is also shortly described, 

 but the account of the development after hatching is 

 altogether much less complete than that of the embryonic 

 stages. 



Grassi (4) was the next investigator to take up the stud}^ of 

 the Cluetognntha, nnd his account published in 1883 gives a 

 very full if not always satisfactory description of the anatomy 

 of the adult, but no new observations of inijiortajico were 



