102 BULLETIN OF THE 
cle, from the fifth to the thirteenth inclusive, contract at their ventral 
ends into cell-strands (Zellenstrange), which extend ventrally and ter- 
minate blindly in an indifferent cell-mass, the limb-bud. Van Bemmelen 
is convinced that these cell-strands, or myotome-buds, give rise to the 
musculature of the fore limbs, but he is unable to determine the further 
process by which this is effected. Similar contributions of mesoderm are 
made by the myotomes in the region of the hind limbs. Each myotome 
is said to develop only one bud, whose cells at its ventral end stain more 
deeply than the surrounding cells of the embryonal tissue, and later are 
entirely lost in the tissue of the limb-bud. 
IV. THe AxiaAL AND ParRieTaL MESODERM. 
(a) Formation of the Primitive Layers, and the Differentiation of Proto- 
vertebre und Lateral Plates. — With the temperature of the water at an 
average of 20° C., the blastopore in the embryo of Fundulus hetero- 
clitus closes during the period between forty-six and forty-eight hours 
after fertilization of the ovum. If in an embryo in which the blasto- 
pore is not yet closed, a transverse section be taken through the region 
indicated by the line at 5 in Figure 1 (Plate I.), the condition of 
the primitive embryonic layers will be shown. From such a section 
(Fig. 5) it can be observed that the spinal cord (cd. sp.) is not yet fully 
formed, and that the superficial layer of the ectoderm (ec’drm.) is not 
involved to form later the medullary canal (can. med., Plate V. Fig. 30) ; 
hence in this particular the development of the spinal cord in Fun- 
dulus differs from that in Amphibians, Elasmobranchs, and, according 
to recorded observations, in some Bony Fishes. 
The endoderm (en’drm., Fig. 5) is already clearly differentiated, and 
lies directly upon the yolk, extending as a single-cell layer from the 
keel laterally to the ectoderm ; hence it comes in contact with the ecto- 
derm both in the axial region of the embryo, ventrad of the keel, and 
also at its extreme parietal margin. 
The mesoderm appears as a band of cells between the ectoderm and 
endoderm, extending longitudinally along either side of the embryonic 
keel. In a transverse section (ms’drm., Fig. 5) the mesoderm has the 
appearance of a layer of cells which is somewhat thickened in the mid- 
dle, while it gradually becomes thinner toward the axial and lateral 
margins. Ina stage of fifty-two hours (Fig. 2) the embryo has become 
relatively much elongated, the optic vesicles (vs. opt.) are already quite 
prominent, Kupffer’s vesicle (vs. A.) can be distinctly observed in a 
