The works of Francis Maitland Balfour, Volume 1 (of 4)

by Francis M. Balfour

With reference to the very first developmental phenomena my observations are confined to two stages during the segmentation[448]. In the earliest of these the segmentation was about half completed, in the later one it was nearly over. My observations on these stages bear out generally the statements of Kupffer and Benecke. In the second of them the blastoderm was already imperfectly divided into two layers—a superficial epiblastic layer formed of a single row of cells, and a layer below this several rows deep. Below this layer fresh segments were obviously being added to the blastoderm from the subjacent yolk.

Between the second of these blastoderms and my next stage there is a considerable gap. The medullary plate is just established, and is marked by a shallow groove which becomes deeper in front. A section through the embryo is represented in Pl. 29, Series A, fig. 1. In this figure there may be seen the thickened medullary plate with a shallow medullary groove, below which are two independent plates of mesoblast (me.p.), one on each side of the middle line, very imperfectly divided into somatopleuric and splanchnopleuric layers. Below the mesoblast is a continuous layer of hypoblast (hy.), which develops a rod-like thickening along the axial line (ch.). This rod becomes in the next stage the notochord. Although this embryo is not well preserved I feel very confident in asserting the continuity of the notochord with the hypoblast at this stage.

At the hind end of the embryo is placed a thickened ridge of tissue which continues the embryonic axis. In this ridge all the layers coalesce, and I therefore take it to be equivalent to the primitive streak of the avian blastoderm. It is somewhat triangular in shape, with the apex directed backward, the broad base placed in front.

At the junction between the primitive streak and the blastoderm is situated a passage, open at both extremities, leading from the upper surface of the blastoderm obliquely forwards to the lower.

The dorsal and anterior wall of this passage is formed of a distinct epithelial layer, continuous at its upper extremity with the epiblast, and at its lower with the notochordal plate, so that it forms a layer of cells connecting together the epiblast and hypoblast. The hinder and lower wall of the passage is formed by the cells of the primitive streak, which only assume a columnar form near the dorsal opening of the passage (vide fig. 4). This passage is clearly the blind sac of Kupffer and Benecke, who, if I am not mistaken, have overlooked its lower opening. As I hope to show in the sequel, it is also the equivalent of the neurenteric passage, which connects the neural and alimentary canals in the Ichthyopsida, and therefore represents the blastopore of Amphioxus, Amphibians, &c.

Series A, figs. 2, 3, 4, 5, illustrate the features of the passage and its relation to the embryo.

Fig. 2 passes through the ventral opening of the passage. The notochordal plate (ch´.) is vaulted over the opening, and on the left side is continuous with the mesoblast as well as the hypoblast. Figs. 3 and 4 are taken through the middle part of the passage (ne.), which is bounded above by a continuation of the notochordal plate, and below by the tissue of the primitive streak. The hypoblast (hy.), in the middle line, is imperfectly fused with the mesoblast of the primitive streak, which is now continuous across the middle line. The medullary groove has disappeared, but the medullary plate (mp.) is quite distinct.

In fig. 5 is seen the dorsal opening of the passage (ne.). If a section behind this had been figured, as is done for the next series (B), it would have passed through the primitive streak, and, as in the chick, all the layers would have been fused together. The epiblast in the primitive streak completely coalesces with the mesoblast; but the hypoblast, though attached to the other layers in the middle line, can always be traced as a distinct stratum.

Fig. B is a surface view of my next oldest embryo. The medullary groove has become much deeper, especially in front. Behind it widens out to form a space equivalent to the sinus rhomboidalis of the embryo bird. The amnion forms a small fold covering over the cephalic extremity of the embryo, which is deeply embedded in the yolk. Some somites (protovertebræ) were probably present, but this could not be made out in the opaque embryo.