Boyer. — Mesoderm in Teleosts. 
Fig. 
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52. 
53. 
58. 
PLATE VIII. 
Transverse section through the posterior region of the pectoral plate of 
an embryo of 43 days, being 14 days older than the one represented 
in Plate VII. This section is from the region corresponding to Fig. 
50, Plate VII., and shows a more complete histological connection 
between pectoral plate and myotome. X 288. 
Transverse section, sixth section back of the one represented in Fig. 51. 
This section is from the posterior region of the pectoral plate. > 288. 
Longitudinal section from the pectoral plate of an embryo of 5 days. 
The section passes between the vertebral region and the longitudinal 
ectodermic fold. The outer layer of the ectoderm is unmodified and 
the lower only slightly modified. The pectoral plate (/a. pet.) extends 
forward to the lateral blood-vessel (va. sun.), and backward it tapers 
into the single-cell layer of the somatopleure. X 288. 
Transverse section through the pectoral plate of an embryo of 5 days, — 
but slightly more advanced than the one represented in Fig. 55,— 
showing the beginning of the ectodermal fold (p/i. ec’drm.) and the 
modification of both layers of the ectoderm. Two nuclei (n/.!) in the 
cuticular layer are shown which still retain in section their character- 
istic spindular form. X 500. 
Transverse section from the pectoral plate of an embryo of about 54 
days. The ectodermic fold (pli. ec’drm.) is more marked than in 
Fig. 54, and the mesodermal elements are beginning to enter it. Two 
nuclei (a, b) are at the base of the space included between the two 
layers of the folded ectoderm. X 750. 
An oblique frontal section from the left pectoral region of an embryo of 
about 44 days. The plane of the section is higher in the axial region 
than in the lateral region. It shows the marginal vein (vn. marg.) 
with contained blood corpuscles, and the relation of the vein to the 
first four protovertebre and to the pectoral plate. The auditory vesi- 
cle (au.) and the posterior gill-pocket are shown directly in front of 
the marginal vein. The pectoral plate, excepting its proximal region, 
is too high to show in the plane of this section. X 65. 
A transverse section through the right pectoral plate of an embryo of 6 
days. The pectoral plate is beginning to shorten in the antero-poste- 
rior direction, and its contour is beginning to extend outward to form 
the limb-bud. In relation to the pectoral plate, the marginal vein 
(va. san.) lies father back and in a more ventral plane than in former 
stages. (Compare va. san., Fig. 53.) 65. 
A section through the limb-bud of the pectoral fin on the right side of an 
embryo of 7 days. The contour of the limb-bud has been carried 
some distance beyond the general surface of the embryo. The 
mesoderm has entered and closely packed all parts of the limb-bud 
excepting the distal tip of the ectodermal fold. The marginal vein 
has moved posteriorly and ventrally, and now lies directly below the 
limb-bud. (Compare va. san. of Figs. 53, 57, and 58.) X 69. 
