THE FOEMATION OF THE LAYERS IH AMPHIOXHS. 339 
Of course Hubreclit may reply that there is no a priori 
impossibility in the formation of a new month. There is cer- 
tainly no analogy for it, and if it is legitimate morphological 
reasoning to assume such changes of function as are implied 
in it, no valid objection can be brought against the attempts 
to evoke a Vertebrate from an Arachnid like Limulus, in 
which Patten and Gaskell display such diabolically brilliant 
ingenuity. 
The general conclusion of our study may be summed up thus : 
The process of gastrulation in Amphioxus leads to the 
formation of a single laye'' of invaginated cells, which there 
is no valid reason for analysing into two kinds. 
The closure of the blastopore in Amphioxus is due to the 
concrescence of the lateral lips of the blastopore and to the 
upgi'owth of the ventral one. By these processes the venti’al, 
not the dorsal surface of the embryo is formed. 
The mesoderm owes its origin to the outgrowth of five 
coelomic pouches from the archenteron in the same manner 
as the coelom of Balanoglossus. 
The formation of the layers in other Vertebrata can be 
derived from that of Amphioxus by allowing for first the 
disturbing influence of the accumulation of food-yolk in the 
ventral wall of the archenteron, and, secondly, in Mammalia 
the disturbing effect of contact with the maternal uterus. 
In starting with Mammalia, and reading their complicated 
pi’ocesses into the development of lower Vertebrata, Pi’ofessor 
Hubreclit has read the book of Vertebrate development 
upside down. 
McGill University, 
April, 1909. 
List op Works referred to in this Paper. 
[Except in the case of Amphioxus this list makes no pretence at 
completeness. In the case of the higher veiLehrates I have given 
references only to the newest work on the subject w'hich I could find.] 
1. Agar. — “ The Development of Anterior Mesoderm in Lepidosiren 
and Protopterus,” ‘ Trans. Roy. Soc. Edin.,’ vol. xlv, pt. 3, 1907. 
