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nective plus about 47!/,° of the remaining length) is derived from 
tissue situated in front of the original position of the dorsal lip; and 
the posterior part (about 72!/,°) is derived from tissue produced by 
the expanding ectoderm during the closure of the blastopore — a con- 
clusion which, it is needless to say, has been reached by several ob- 
servers on the strength of various kinds of evidence. 
We fall into a tangle of unverifiable conjectures, when we try to 
go deeper than this general conclusion in the endeavor to ascertain 
what was the precise situation of the neural plate material when the 
embryo was in the blastula stage; or at the time when (in any one 
region) the existence of the randzone came to an end, and the ecto- 
derm (or as it might without chance of confusion be called, the micro- 
mere layer) and primitive entoderm (yolk) became sharply separated. 
For direct observation shows that the cells of at any rate the peri- 
pheral region of the micromere layer are continually shifting their po- 
sition as the whole layer expands. Cells at some distance from the 
edge of the layer approach the edge, and cells at the edge pass in- 
wards. Thus while the micromere layer gives rise to the ectoderm, 
it apparently also gives rise to a part of the lining of the arch- 
enteron, as Lworr (Bull. de la Soc. imper. des Naturalistes de Moscou, 
T. 8, 1894) maintained some years ago. Hence cells which at one 
time lay in front of the dorsal lip in a region which was destined 
to form a part of the neural plate, may have moved entirely out of 
that region before the histological differentiation of the plate was begun. 
Conversely, while during the contraction of the blastopore some of 
the material of the lip must become incorporated in the neural plate 
region, it would seem to be a fact that the cells so incorporated 
originally lay at a considerable distance from the blastopore lip when 
the latter was first marked out. 
University of North Carolina, Chapel Hill N.C., U.S.A. 
June 25, 1901. 
