ON THR DEVELOPMENT OF SAGITTA. 357 



the surface. They are enclosed in a tinn transparent shell, 

 which may almost fit the egg, as in S. enflata, or may leave 

 a large space containing fluid in which the egg is suspended 

 (S. bipunctata). 



O. Hertwig's account (3) of the early embryonic develop- 

 ment, as seen in living embryos, is so complete and accurate 

 that it is unnecessary for me to go over it in detail. I con- 

 firmed in all points his account of the cleavage, gastrulation, 

 and formation of the archenteric folds, and further the closing 

 of the blastopore and origin of the mouth, and found that the 

 genital cells appear m the archenteron as he describes. They 

 lie slightly either dorsally or ventrally of the middle line, but 

 I was not able to determine on which side. This eccentricity 

 is shown in one of Hertwig's figures, but not mentioned in 

 his text. In one important point, however, my observations 

 differ from his, and confirm the account of the development 

 given by Biitschli (2). After the mouth has been formed 

 the two lateral lobes of the archenteron become much 

 restricted by the narrowing of the embryo, and their anterior 

 ends are then separated off as distinct cavities by the meeting 

 and fusion of their walls. The two anterior ''head-cavities" 

 so produced lie at the sides of the pharynx (PI. 19, figs. 3—5), 

 and are from the first very small and soon become obliterated ; 

 but the mesoblast enclosing them persists, separated off from 

 the remainder, and gives rise to the mesodermal structures of 

 the head. The size of the cavities varies in different species 

 according to whether the egg-shell is large and leaves plenty 

 of room, or the reverse. In S. bipunctata they are easily 

 recognisable, while in S. enflata they are from the first 

 mere dots, and might easily be overlooked were not other 

 species available for comparison (PI. 19, fig. 4). 



The backward growth of the folds and displacement of the 

 genital cells into the coelomic cavities takes place as Hertwig 

 describes, and at this stage a faint line can sometimes be 

 seen running back on the dorsal wall of the archenteron from 

 the free end of the folds to the point where the blastopore 

 closed. This looks in the living embryo rather as if the 



