592 E. W. MACBRIDE. 



oxus, and which was regarded by Lankester as a forward pro- 

 longation of the atrial folds, is a downgrowth of the upper 

 margin of the praeoral pit as far as its upper part is con- 

 cernedj its lower part being formed beneath the mouth inde- 

 pendently. 



Lwoff's paper (10) deserves special attention not only 

 because it is the first systematic study of the embryology of 

 Amphioxus with modern methods, but also on account of the 

 disagreement of the results he arrived at with those of Hat- 

 schek. LwofF maintains that before invagination commences 

 the cells constituting the blastula become sharply divided into 

 two sorts, endoderm and ectoderm cells ; that invagination 

 first involves the endoderm cells, but that later the ectoderm 

 bends round the dorsal lip of the blastopore, and displacing 

 the endoderm forms the dorsal wall of the gut, and that the 

 whole of this ectodermal stage is employed in the formation 

 of the notochord, and in the production of mesoderm. Lwoff 

 believes that the mesodermal " folds " are the mechanical result 

 of the pressure of the nerve-tube and notochord on the upper 

 wall of the gut; he asserts that the cavity of this fold dis- 

 appears, and that the cavities of the somites appearing later 

 have no connection with or relation to the enteric cavity ; 

 hence that Amphioxus is not an '' enterocoelous animal." He 

 finds that the pole-cells of Hatschek have no existence, therein 

 confirming Wilson (14) ; and that the blastopore is closed not 

 by the special growth of the dorsal lip, but from all sides. 



It will thus be evident that the most interesting part of the 

 development of Amphioxus, viz. the formation of the primitive 

 germinal layers, was involved in great uncertainty, and it was 

 with a view of clearing up the questions thus raised, and also 

 from a feeling of dissatisfaction with the accounts given of the 

 nature and origin of such structures as Hatschek's nephri- 

 dium, and the cavities in the atrial (metapleural) folds, &c., 

 that I was led to undertake a re-investigation of the whole 

 subject. That the early development of Amphioxus is of 

 great theoretical importance can, I think, be hardly denied ; 

 when we consider that in it we have the only instance of 



