GASTRULATION IN BIRDS. 
157 
lying between tlie separated halves of the neural tube. Tlie 
notochord is perfect and to one side, the entoderm is uninjured. 
May not the defect in tlie neural tube be simply mechanical, 
due to the pressure of an extra-ovate which became separated 
off from the edge of the blastoderm as a result of the 
cauterising, and which, passing into the area pellucida between 
the blastoderm and vitelline membrane, caused the injury 
seen ? Text-fig. 19 strongly suggests this solution. 
Exp. VII and VIII. The figures do not enable one to 
appreciate the character of the defects. Sections ai’e not 
given. 
The remaining experiments, IX-XIII, were made upon the 
blastoderm after the eggs were laid, and therefore after 
Patterson’s supposed concrescence of the lip must have been 
completed, because by now, according to him, the main axial 
line of the embryo produced by this concrescence is entirely 
enclosed within the blastoderm margin, aud there are no longer 
any free blastoporic lips that could come together. 
Although Patterson still speaks of concrescence, Exji. 
XI, p. 115, it clearly cannot be a phenomenon similar to that 
which, as he alleges, occurs during gastrulation. One is 
inclined in this particular connection to say with Professor 
MacBride (re “ Amphioxus,” ‘Quart. Journ. Micr. Sci.,’ vol. 
liv, p. 302): “Of course in every structure there is an 
imaginary middle line, and if anyone chooses to say that 
this band of dividing cells consists of right and left halves 
wliicli unite together as quickly as they grow, I shall not 
waste lime in arguing against such a metaphysical conception, 
which is capable neither of proof nor disproof.” 
From Patterson’s final discussion it is clear that he quite 
fails to appreciate the distinction between gastrulation and 
snbsecpient growth in length. It is not really true that “all 
of the chorda and mesoderm are derived from the primary 
invaginated layer ” in Amphioxus. The anterior part is so 
derived, but the posterior part is derived from the proliferating 
lips of the blastopore, which can be described neither as ecto- 
derm nor endoderm. 
