[Figure 13c] passes through the anterior part of the heart about seventy-five sections posterior to [Figure 13b]. The embryo in this region is narrow but deep (dorso-ventrally), the depth being largely due to the size of the heart. The ectoderm (ec) is considerably thickened on each side of the pharynx (ph); this thickened area may be traced for some distance both anteriorly and posteriorly from this point; its significance could not be determined. The spinal cord (sc) and notochord (nt) need no special description; the former is smaller and the latter larger than in the more anterior sections. The two large blood-vessels (ac) near the spinal cord and notochord are probably the anterior cardinal veins. The aortæ are cut by the plane of this section just anterior to their point of fusion into a single vessel. A few blood corpuscles are seen in the right aorta. The enteron (ent), cut posterior to the region of the gill clefts, is a large elliptical cavity, with its long axis in a transverse position. Its entodermal wall is comparatively thin and smooth, with the cell nuclei arranged chiefly on the outer side, i. e., away from the cavity of the enteron. The body cavity (bc) is here still unenclosed, and its walls, the somatic stalk, are cut off close to the body of the embryo. The heart (ht), the most conspicuous feature of this section, is nearly as large in cross-section as all the rest of the embryo. As seen in such a section it is entirely detached from the body of the embryo, and in this particular case has about the shape of the human stomach. The mesoblastic portion of its wall (mes′) is of very irregular thickness; it forms a dense layer entirely around the outside, except for the pointed dorsal region, and is especially thick along the ventral margin, where it is thrown into well marked folds, the heavy muscle columns. Lining the cavity of the heart is the membranous endothelium (en′), and between this and the dense outer wall just described is a loose reticular tissue with but few nuclei.
As the series is followed toward the tail the sections diminish in size until, at a point about one third the embryo length from the posterior end, they are of scarcely one fourth the area of the sections through the region of the hindbrain.
[Figure 13d] is about one hundred and twenty-five sections posterior to [Figure 13c]. Although not so small as the sections that follow it, this section is considerably smaller in area than the one last described. The amnion (a), which was not represented in the last three figures, is very evident here. The spinal cord (sc) is considerably smaller here than in the preceding figure, while the notochord (nt) is not only relatively but actually larger than in the more anterior regions. Beneath the notochord is the aorta (ao), now a single large vessel. The mesoblast on each side of the body is here differentiated into a distinct muscle plate (mp). These muscle plates have very much the appearance of the thickened ectoderm seen in the younger stages of development. At about its middle region (i. e., at the end of the reference line ec) each muscle plate is separated from the overlying ectoderm by an empty space; this space is still more marked in some other series. Ventral to the aorta, and supported by a well marked though still thick mesentery (ms), is the intestine. It is a small, nearly cylindrical tube with thick walls; the splanchnic mesoblast which surrounds it is more dense than the general mass of mesoblast; it was somewhat torn in the section and is so represented in the figure. The urinary organs have made considerable progress since the last stage. In the figure under discussion they are seen as a group of tubules on either side of the aorta. The tubule most distant from the middle line, on each side, is the Wolffian duct (wd). It extends through the posterior two thirds of the embryo and varies in diameter at different points; it is usually lined with a single layer of cubical cells which contain large nuclei. The Wolffian bodies (wt) are a mass of slightly convoluted tubules that may be traced throughout the greater part of the region through which the Wolffian duct extends. These tubules also vary somewhat in diameter, but they are usually of greater caliber than the duct. No actual nephrostomes are to be seen, though the occasional fusion of a tubule with the peritoneal epithelium, as is seen on the left side of the present figure, may represent such an opening.
[Figure 13e] is about one hundred and forty sections posterior to the section just described. The embryo is here very slender, so that the contrast between this and the first figure ([13a]) of this stage is remarkable. Except in size, this section does not differ greatly from the preceding. The spinal cord, notochord, etc., are smaller than before, but are of about the same relative size. The mesentery (ms) in the section drawn was torn across, so that the intestine is not represented. Medial to the Wolffian duct is a tubule (wt), which seems to be the same as those which were called Wolffian tubules in the preceding stage, but which may be the beginning of the ureter.
[Figure 13f], about two hundred and fifty sections posterior to the last, passes through the extreme posterior end of the embryo. The section is nearly circular in outline and is somewhat larger than the preceding. The amnion (a) completely encircles the embryo. The ectoderm (ec) is of fairly even thickness, and the mesoblast which it encloses is of the usual character. The spinal cord (sc) is nearly circular in outline, as is its central canal. The digestive tract (ent) is larger in section than it was in more anterior regions; it is nearly circular in cross-section and its walls are made up of several layers of cells, so that it resembles to a considerable degree the spinal cord of the same region. In the narrow space between the spinal cord and the hindgut is seen the notochord (nt), somewhat flattened and relatively and actually smaller than in the preceding figure. A few scattered blood-vessels may be seen in the mesoblast at various places.
A sagittal section of an embryo of this stage, drawn under the same magnification as were the transverse sections, is shown in [Figure 13g]. The embryo being bent laterally could not be cut by any one plane throughout its entire length, so that only the anterior end is represented in the figure. The amnion (a) may be clearly seen except at certain places where it is closely adherent to the superficial ectoderm. Under the low magnification used the superficial ectoderm cannot be distinguished from the ectoderm of the nervous system. The plane of the section being in the anterior end almost exactly median, this part of the central nervous system is seen as the usual retort-shaped cavity, while in the region back of the brain, where the neural canal is narrow, the section passes through the wall of the spinal cord (sc) and does not show the neural canal at all. The wall of the forebrain (fb) is quite thick, especially at the extreme anterior end; the wall of the midbrain (mb), where the marked cranial flexure takes place, is somewhat thinner, and it gradually becomes still thinner as it is followed posteriorly over the hindbrain (hb). Between the floors of the fore- and hindbrains, in the acute angle caused by the cranial flexure, is the anterior end of the notochord (nt), the only part of that structure that lies in the plane of the section. Ventral and posterior to the notochord is a large cavity, the pharynx (ph), whose entoblastic lining can scarcely be distinguished under this magnification from the surrounding tissues. The stomodeal opening being as yet unformed, the pharynx is closed anteriorly; posteriorly also, owing to the plane of the section, the pharynx appears to be closed, since its connection with the yolk stalk is not shown. In the floor of the pharynx, almost under the reference line ph, a slight depression marks the position of the first gill cleft. In the mesoblast ventral to the pharynx and near the gill cleft just mentioned, a couple of irregular openings represent the anterior end of the bulbus arteriosus, posterior and ventral to which is the heart (ht), a large, irregular cavity. The dorsal aorta (ao) may be seen as an elongated opening in the mesoblast, extending in this section from the middle region of the pharynx to the posterior end of the figure where it is somewhat torn. Two of the eighteen or twenty pairs of mesoblastic somites possessed by this embryo are shown at the posterior end of the [figure] (s), where the plane of the section was far enough from the median line to cut them.
STAGE XI
Figure 14 ([Plate XXI.])
Only the anterior region of this embryo is shown in the figure, which is a ventro-lateral view. While there is some change in the general shape and in parts of the head, the reason for figuring this stage is to show the first gill cleft (g′), which lies at an acute angle to the long axis of the neck behind the eye (e). The cleft is narrow but sharp and distinct in outline; it shows neither in this nor in the following stages the branched, Y-shaped outline mentioned by Clarke.