1890.] The Mesoderm and the Calom. 887 
tebrates the mesoderm arises on each side, but also in the axis, 
and becomes two masses, when the medullary groove and noto- 
chord appear. In Amphioxus the medullary plate and notochord 
appear very early, and the division of the mesoderm may be due 
to that fact. Amphioxus is undoubtedly a lower type, but 
whether it really preserves the older type of development in its 
purity is doubtful; indeed, it is probably a tunicate rather than a 
vertebrate. 
FD 
s 
3 EO 
g 
U 
Fic. 25.—Amphioxus embryo; after Hatschek. A, side view; B, ventral view. Æc, 
ectoderm; Zn, entoderm; a, neuropore; JV, nervous system ; Mes, mesoderm; Mb, 
mesoblast ; 7-5, segments. 
Hatschek, in a series of brilliant investigations, has shown that 
in many bilaterally symmetrical invertebrates the mesoderm 
arises as two bands of cells, which subsequently divide into a 
series of closed sacks (segments), and which, during their own 
formation, terminate each in a single large posterior cell (meso- 
blast), which throws off cells to add to the mesodermal band 
(germ-band, Keimstreif). This “mesoblast” by its appearance 
and position appears to be a derivative of the entoderm. As a 
matter of speculation, we may assume that in Amphioxus we 
have the germ-bands, but characterized by an exceedingly pre- 
cocious segmentation. We can further assume that in vertebrates 
