GASTRULATION IN BIRDS. 
153 
The injury was agiiin on the edge, but now the edge is a 
lip. In these he Cnds an injury in entoderm only, thei'e- 
i'ore he says there is still an inrolling of entoderm. 
In specimens ‘^slightly older” such experiments show 
injuries in ectoderm and mesoderm but not in the entodeiun, 
showing that the involution has ceased.” 
The whole series of experiments recorded under the head- 
ing ‘'Experiment II ” seems to me to be questionable in the 
extreme. Anyone reading the first two paragraphs of that 
section with a critical mind must perceive how fragile is the 
evidence upon which such far-reaching results are based. 
He writes (p. 93), in desciibing the subsequent effect of an 
injury made to the edge of the lip in the middle dorsal line, 
“ There is no evidence of an injury either in the ectoderm or 
mesoderm, and hence we must conclude that the affected 
cells have been brought to their present position (in the 
entoderm) by an inrolling under the posterior tnargin. 
Although this operation has been repeated several times with 
the above restdts, yet the position of the injury in the ento- 
derm may vary in an anterior posterior direction ; but this 
variation is easily accounted for by the fact that one can tell 
in the living egg only approximately the extent to which 
invagination has progressed.” 
'I'lius wo .see the results obtained are vari.able ; and he goes 
on to say that “ if an injury be made in the same manner as 
above on slightly older blastoderms, the alfected region is 
not found in the entoderm, but in the ectoderm and meso- 
derm, showing that the involution has ceased” (p. 93). 
There is little here in the nature of exact or accurate 
experiment. There are no times^ or measurements given, but 
instead of these ho bases results upon operations performed 
on “slightly older” blastoderms than those the stage of 
development of which “ one can tell only approximately.” 
There is also the difficulty presented by this hypothesis of 
formation of a blastopore lip before the laying of the egg. 
' Some times are yiven in the explanation of the plates. 
