THE EORMATIOX OP THE LAYERS IX AMPHIOXHS. 279 
The Formation of the Layers in Amphioxus and 
its Bearing on the Interpretation of the Early 
Ontogenetic Processes in Other Vertebrates. 
By 
E. W. MacBiide, D.Sr., LE.D., F.R.S., 
Strathcona Professor of Zoology in McGill University, MonUeal. 
With Plates 18, 19, 20, and 21. and 10 Text-figures. 
The reader.s of the Quarterly .Journal of Micro- 
scopical Science have been treated in a recent number to 
a brilliant essay on the early ontogenetic stages of vertebrates 
from the well-known pen of Professor Hubrecht (18), who 
has dealt with the subject from the point of view of a 
specialist in mammalian embryology, and may be said to 
have envisaged the development of vertebrates through 
mammalian spectacles. 
To every question there are at least two points of view, 
and I desire in what follows to lay before the readers of this 
journal another way of looking at the early development of 
Vertebrata, taking as my starting-point, not the highest 
members of the group, as Professor Hubrecht has done, but 
the lowest form which all zoologists are agreed in including 
within the phylum Vertebrata, viz. Amphioxus. Which of 
the two points of view will ultimately gain the greatest 
amount of support will depend, I am convinced, upon which 
offers the simplest and most natural phylogenetic explana- 
tion of the facts to be interpreted. 
The first part of this paper will therefore consist of a 
re-description of the formation of the layers in Amphioxus. 
I say a re-description, because eleven years ago I g’ave a 
VOL. 54, PART 3. NEW SERIES. 21 
