THE FOEMATION OF THE LAYEES IN AMPHIOXUS. 
317 
slowly advances to meet the dorsal lip. This latter, after the 
invagination has taken place, has grown somewhat in 
length, and therefore in its edge as in the dorsal lip 
of the blastopore inAmphioxus is situated a grow- 
ing point by which above new ectoderm and below 
new endoderm are produced. This has been proved up 
to the hilt by Assheton in his most interesting experimental 
studies (2). In Siredon the ventral part of the blastopore 
remains open as the anus, whilst the dorsal part is included 
within the medullary groove as the neureuteric canal. The 
Text-fig. 5. 
AMPHIOXUS SIREDON (BRAOHET) STURGEON (deane) 
Sagittal sections through the Imlf completed gastrulae of Amphioxus, 
Siredon, and Sturgeon. 
tail forms as a new growth o n 1 y a f t e r g a s t r u 1 a t i o n is 
complete. It will be seen that, making allowance for the 
difference in the amount of yolk there is an extraordinary 
correspondence between the development of Amphioxus, as 
described in this paper, and that of Siredon. The early 
formation of the dorsal lip and the later up-growth of the 
ventral lip show the same sequence. The difference between 
the two embryos lies in the accumulation of yolk in the 
ventral wall of the archenteron of Siredon, so that the cells 
here are not only more numerous but more massive than those 
forming the dorsal wall, and therefore the difference between 
roof and floor of the archenteron, which is barely indicated in 
