THE FOEMATION OF THE LAYERS IN AMPHIOXUS. 283 
by Basilius Lvvof¥ (25). According to this observer the pro- 
cess oP gastrulatioii consists of two parts ; during the first 
stage there is invaginated the genuine large-celled endoderm, 
consisting of the larger blastoineres situated at the lower pole 
of the blastula; from this the epithelium of the gut is formed. 
During the second stage of gastrulation the small-celled ecto- 
derm is invaginated round the dorsal lip of the blastopore, 
and forms the dorsal wall of the “ archenteron ” or gut of the 
completed gastrula. This ectodermal plate, however, is com- 
pletely used up in the formation of the notochord and of the 
mesodermal folds ; the edges of the endodermal portion of the 
gut meet beneath it, and so complete the definitive gut. The 
cavities of these folds stand in no relation to the future body- 
cavity of the adult, for they are produced by the pressure on 
the dorsal wall of the gut of the in-sinking nerve-plate, which 
is commencing to be invaginated, and their cavities sub- 
sequently disappear entirely, whilst the coelom consists of 
spaces wliich arise subsequently as splits in solid masses of cells. 
It follows that mesoderm and notochord are of ectodermal 
origin, and that Am phioxus is in no sense an “enterocoelous” 
animal, nor are the processes of the formation of its layers 
capable of being analysed into simple processes of folding in 
the wall of an epithelial sac. Lwoff further denies that the 
blastopore closes in a slit-like manner from before backwards. 
Lwoff’s observations seemed to fit into what had been 
observed in the development of the higher Vertebrata. He 
emphasises this point, and ingenuously confesses that he would 
not have felt certain about his account of the development of 
Amphioxus if he had not found it confirmed by his observa- 
tions on the higher Vertebrata. An account, however, so 
radically distinct from that given by Hatschek needed con- 
firmation, and accordingly in the year 1898 four papers 
appeared on the subject by Sobotta (32), Samassa (30), 
Klaatsch (20), and myself (25). Sobotta deals only with the 
gastrulation. He denies Lwoff’s assertion of a difference in 
histological character between the cells forming the dorsal 
wall of the archenteron and those constituting its ventral wall. 
