888 The American Naturalist. [October, 
we have the germ-bands also, but that they are modified, 1°, by 
the loss of the distinct terminal mesoblast; 2°, by precocious 
fusion in the axial line; and 3°, by extremely retarded segmen- 
tation. A great deal may undoubtedly be said in favor of these 
two assumptions, which together constitute the only “ THEORY 
OF THE VERTEBRATE MESODERM” which of the many theories 
which have been advanced is at all likely, in my opinion, to 
prove of permanent value. 
Expansion of the Mesoderm—After the mesoderm is once 
formed as a distinct layer, without connection with the primitive 
layers except in the axial line, it expands independently,—that is, 
by the proliferation of its own cells. During its early expansion 
Fic, 26.—Diagrams of the embryonic area of the chick. Ao, area opaca; Ap, area 
pellucida ; pr, primitiv ve streak; mes, mesoderm. After Duval. 
the mesoderm assumes in all amniota a definite series of charac- 
teristic outlines. It is at first pear-shaped (Fig. 26, A), the 
anterior end being pointed; it extends a short distance only in 
front of the primitive streak, and is widest a little distance behind 
the area pellucida, 45. The same stage is found in mammals 
(see Kölliker, Grundriss, p. 93, and Fig. 71). The condition in 
the chick at about the twentieth hour of incubation is indicated 
by Fig. 26, B, drawn on the same scale as A, and at the close 
of the first day by Fig. 27. In the last-mentioned figure it will 
be noticed that the mesoderm is expanding unequally in front, 
having sent -out two lateral wings, which leave a median space 
between them without mesoderm. These wings continue their 
growth, and finally meet in front, so that in the anterior part of 
the area pellucida there is a small tract without any mesoderm, 
although there is mesoderm all around it. This tract is the pro- 
