ON THE DEVELOFMEN'l' OF SAGITTA. 353 



added on the development. Since his time the only impor- 

 tant work that has been done on the embryology is a note by 

 Jourdain, This must be referred to, because he contradicts 

 the statements of all previous workers as to the formation of 

 the mesoblast, which he says is not formed by archenteric 

 diverticula; but by a process of delamination between the 

 ectoderm and. endoderm. This process is not fully described, 

 and there are no figures, so that his exact meaning is hard to 

 discover. 



It will be seen, therefore, that a number of important ques- 

 tions concerning the Chastognatha have been left unsolved ; 

 with regard to some points nothing was known, while with 

 others observers have arrived at different results. Leaving 

 aside the note by Jourdain, which reopened the whole question 

 of the origin of the mesoderm and the coelom, there was 

 nothing certainly known about the mode of development of 

 the transverse septa or of the oviducts and sperm-ducts, both 

 of which are matters of great importance on account of their 

 bearing on the theory that Sagitta is related to the Annelida. 

 It was important also that Blitschli's observation on the 

 formation of head-cavities at an early stage should be, if pos- 

 sible, confirmed, or at least that his account of the develop- 

 ment should be reconciled with that given by Hertwig. 

 There are also many minor points ; for example, Hertwig in his 

 monograph describes the nervous system as being of two 

 parts, one ectodermal and sensory in function, the other meso- 

 dermal and motor. This idea was founded on the anatomy of 

 the adult, but no attempts have been made hitherto to confirm 

 or deny it by study of the embryology. 



The present work, undertaken to solve if possible these 

 questions, and to throw light on the systematic position of 

 the Chsetognatha, was carried on chiefly at Naples, between 

 October, 1000, and June, 1901, while I occupied the Cambridge 

 table at the Zoological Station. I wish to take this oppor- 

 tunity of acknowledging my indebtedness to Professor Dohrn 

 and all the officers of the station for their unvarying kindness 

 and willingness to give help. 



VOL. 46, FAKT 2. — NEW SERIES. Y 



