Part 4

Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the time of the first appearance of the medullary groove. (Magnified 300 diam.)

The specimen is taken from a portion of the blastoderm which will form part of the embryo. It shews two large nuclei of the yolk (n) and the network in the yolk between them; this network is seen to be closer around the nuclei than in the intervening space. The specimen further shews that there are no areas representing cells around the nuclei.

Fig. 3. Section parallel with the long axis of the embryo through a blastoderm, in which the floor of the segmentation cavity is not yet covered by a complete layer of cells. (Magnified 60 diam.)

It illustrates (1) the characters of the epiblast, (2) the embryonic swelling (es), (3) the segmentation cavity (sg). It should have been drawn upon the same scale as fig. 4; the line above it represents its true length upon this scale.

Fig. 4. Longitudinal section through a blastoderm at the time of the first appearance of the embryonic rim, and before the formation of the medullary groove. (Magnified 45 diam.)

It illustrates (1) the embryonic rim, (2) the continuity of epiblast and hypoblast at edge of this, (3) the continual differentiation of the lower layer cells, to form, on the one hand, the hypoblast, which is continuous with the epiblast, and on the other the mesoblast, between this and the epiblast; (4) the segmentation cavity, whose floor of cells is now completed.

N.B. The cells at the embryonic end of the blastoderm have been made rather too large.

Fig. 5. Surface view of the blastoderm shortly after the appearance of the medullary groove. To shew the relation of the embryo to the blastoderm.

Fig. 6a and b. Two transverse sections of the same embryo, shortly after the appearance of the medullary groove. (Magnified 96 diam.)

a. In the region of the groove. It shews (1) the two masses of mesoblast on each side, and the deficiency of the mesoblast underneath the medullary groove; (2) the commencement of the closing in of the alimentary canal below, chiefly from cells (na) derived from the yolk.

b. Section in the region of the head where the medullary groove is deficient, other points as above.

Fig. 7a and b. Two transverse sections of an embryo about the age or rather younger than that represented in fig. 5. (Magnified 96 diam.)

a. Section nearer the tail; it shews the thickening of the hypoblast to form the notochord (ch´).

In b the thickening has become completely separated from the hypoblast as the notochord. In a the epiblast and hypoblast are continuous at the edge of the section, owing to the section passing through the embryonic rim.

Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (1) the flattened head (h) where the medullary groove is deficient, (2) the caudal lobes, with a groove between them; it also shews that at this point, the medullary groove has become roofed over and converted into a canal.

Fig. 8a. Transverse section of fig. 8, passing through the line a. (Magnified 90 diam.) The section shews (1) the absence of the medullary groove in the head and the medullary folds turning down at this time instead of upwards; (2) the presence of the pleuro-peritoneal cavity in the head (pp); (3) the completely closed alimentary canal (al).

Fig. 8b. Transverse section of fig. 8, through the line b. (Magnified 90 diam.) It shews (1) the neural canal completely formed; (2) the vertebral plates of mesoblast not yet split up into somatopleure and splanchnopleure.

Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object a little older than the embryo represented in fig. 8. (Magnified 20 diam.) The internal anatomy has hardly altered, with the exception of the medullary folds having closed over above the head and the whole embryo having become more folded off from the germ.

The two caudal lobes, and the very marked groove between them, are seen at ts. The front end of the notochord became indistinct, and I could not see its exact termination. The epithelium of the alimentary canal (al) is seen closely underlying the notochord and becoming continuous with the epiblast at the hind end of the notochord.

The first visceral cleft (1vc) and eye (op) are just commencing to be formed, and the cranial flexure has just appeared.

Fig. 10. Section through the dorsal region of an embryo somewhat older than the one represented in fig. 9. (Magnified 96 diam.)

It shews (1) the formation by a pinching off from the top of the alimentary canal of a peculiar body which underlies the notochord (x); (2) the primitive extension of the pleuro-peritoneal cavity up to the top of the vertebral plates.

PLATE 4.

Fig. 11a, b, and c. Three sections closely following each other from an embryo in which three visceral clefts are present; a is the most anterior of the three. (Magnified 96 diam.) In all of these the muscle-plates are shewn at mp. They have become separated from the lateral plates in b and c, but are still continuous with them in a. The early formed mass of muscles is also shewn in all the figures (mp´).

The figures further shew (1) the formation of the spinal nerves (spn) as small bodies of cells closely applied to the upper and outer edge of the neural canal.

(2) The commencing formation of the cells which form the axial skeleton from the inner (splanchnopleuric) layer of the muscle-plate. Sections b and c are given more especially to shew the mode of formation of the oviduct (ov).

In b it is seen as a solid knob (ov), arising from the point where the somatopleure and splanchnopleure unite, and in c (the section behind b) as a solid rod (ov) closely applied to the epiblast, which has grown backwards from the knob seen in b.

N.B. In all three sections only one side is completed.