THE EOEMATIOX OF THE LAYERS IN AJIPHIOXUS. 289 
another embryo wliicli lie describes development is mucli 
more advanced, and quite a number ol somites have been 
formed. Nevertheless, not only is there a wide blastopore 
found at the posterior end of the animal, but also the same 
ventral diverticulum of the archenteron, and there is further 
evidence to be found that the front part of the blastopore 
has healed by “concrescence.” Legros regards his observa- 
tions as proving that Lvvott and Cerfontaine are right in 
supposing that the roof of the archenteron is ectodermic ; 
and the horizontal ridges he regards as conveniently marking 
the boundaries of the ectodermal and endodermal portions of 
this sac; and he naturally regards his older embryo as 
])roving that the blastopore closes by concrescence. It must, 
however, be remembered that in normal development the 
blastopore is reduced to its normal dimensions before any 
trace of mesodermal grooves or somites is formed, and the 
processes by which a pathological embryo adjusts itself to 
the normal condition of affairs are likely to be widely 
different from what occurs in the normal course of develop- 
ment. This is obvious even from Legros’s figures; for if 
concrescence took place normally the tissue destined to form 
the notochord would be formed by the meeting in the middle 
line of two svmmetrical halves ; now, in Leyros’s later embrvo 
the notochord is seen to belong entirely to one lip of the two 
which meet in the middle line. Further, in a normal 
Amphioxus so soon as the mesodermal grooves have been 
formed a great growth in length takes place largely 
localised in the hinder part of the embryo ; now, if through 
any reason this growth was unequal we should at once get 
Legros’s type of embryo without having any light thrown ou 
what took place in normal embryos. Legros’s abnormalities 
are, therefore, probably not due to disturbances in the pro- 
cesses of formation of the layers, but to disturbances in the 
process of growth in length which supervenes very early in 
Amphio.xus. Legros agrees with Cerfontaine that Amphi- 
oxus is truly an enterocoelous animal, for he finds that the first 
pair of somites long retain their open connection with the 
