OASTUIILATION IN BlUnS. 
155 
well known to be below tins roof, i. e. they are part of the 
ai’cbenteron. 
Exp. III. (Operation 34^ hours after fertilisation. Subse- 
quent incubation 34 hours.) 
From the plan of his text-fig. 16 one is bound to conclude 
that the stage does not materially differ from the stage of the 
preceding experiment, his text-fig. 10. Each shows a similar 
diameter, a similar forward extension of the endoderm, a 
similar width of what he regards as blastopore opening. 'I’he 
only difference is that in text-fig. 10 the dorsal lip of the 
“ blastopore ” is slightly convex, in text-fig. 1 6 slightly concave 
in surface view. 
In Exp. II an injuiy was made on the edge, and the result 
was a defect in the endoderm at a spot posterior to the 
nineteenth somite. 
In Exp. Ill an injury was made on the surface just within 
the margin. The difference in position of the injury would 
appear to be not more than the diameter of the needle used. 
Result, a defect in the region of the head-fold. Therefore the 
difference in position of less than a needle’s diameter in the 
maiking of a blastoderm corresponds with a difference in 
the embryo which includes the greater part of the body. If 
this is so we must despair of getting anything approaching 
accurate results by such methods. 
Patterson lihewise thinks it unlikely that this small area 
should give rise to .so much embryo directly, and assumes, as 
we have seen, that there is a concrescence. 
Exp. IV. (Operation 34| hours after fertilisation. Subse- 
quent incubation for 36| hours.) 
On a blastoderm similar to Exp. Ill a spot was marked on 
the margin 10° to the right of the middle line so close to 
the margin that the outer surface of the needle was level 
with it. Result, a defect “ on the right neural fold in the mid- 
brain region.” 
If the main axis of the embryo is formed by coalescence of 
the two gerrn-rings, surely, then, it is in the median plane that 
the injury should be found, i. e. the ventral wall of the neural 
