This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2114). Towards the close of the first day of incubation the blastoderm has already begun to change its appearance, and two white filaments are apparent in the middle of the central pellucid circle. Supposing a longitudinal section of it at this period, the membrana vitelli will be found to have become more prominent where it passes over the germinal space (fig. 372,1, d.) The outer layer of the blastoderm (c) has become thickened at e into the first rudiment of the dorsal portion of the embryo; but the mucous layer (a) and the vascular layer (b) have as yet undergone little alteration.
(2115). At the commencement of the second day (fig. 372,2), the anterior portion of the embryo is dilated, and bent down so as to inflect the three membranes of the blastoderm at this point.
(2116). At the conclusion of the second day this inflection is carried still further, and from the vascular layer a single pulsating cavity (fig. 372,3, h), the punctum saliens (the first appearance of a heart), has become developed; so that considerable advance is already made towards that disposition of the foetus and its membranous investments represented in the next figure, to which we now beg the reader's attention.
(2117). The serous membrane (fig.372, c) has at the third day become reflected to a considerable distance over the back of the foetus; at one extremity investing the head with a serous covering, while at the opposite it in like manner covers the tail: it is this reflection of the serous layer which forms the amnion, as will be observed in fig. 373, where the amniotic sac (c) is completed.
Fig. 373. Embryo in a more advanced stage.
(2118). The mucous layer (a) is now seen to line the as yet open space which is to form the abdominal cavity, and by its inflections gives birth to the rudiments of the abdominal viscera.
(2119). From the vascular layer (b) has been developed the heart, now composed of two chambers (a, b), and the branchial arteries (c), which join to form the aorta (m), exactly as in the Menojpoma (fig. 343.) The allantois (jp), the uses of which will be described hereafter, likewise begins to make its appearance*.
(2120). At the fifth day (fig. 374) the lineaments of the viscera become tolerably distinct. The sac of the amnion (c) is completed; the liver (i) and the lungs (e) begin to show themselves; and the bag of the allantois (p) is largely developed; still, however, the heart (a, b) is that of a fish, and the aorta (m) formed by the union of the branchial arches (c); so we have yet to trace how, as the lungs increase in size, the circulatory apparatus becomes changed and the branchial organs obliterated.
* Des Branchies et des Vaisseaux branchiaux dans les Embryons des Animaux vertebres, par Prof. Ch. Ernst v. Baer (Ann. des Sci. Nat. torn. xv.).
Fig. 374. Embryo about the fifth day of incubation.
(2121). On the third day of incubation there exist four vascular arches (fig. 373, c) on each side, having a common origin from the bulb (b), which obviously represents the bulbus arteriosus of Fishes and Reptiles, before described; these encircle the neck, and join on arriving in the dorsal region to form the aorta, which commences by two roots, each made up of the union of the four branchial vessels of the corresponding side. The vascular arches are developed one after the other, the most anterior being visible even on the second day; shortly, a second appears behind the first, the former in the meantime becoming considerably larger; and at length the third and the fourth are formed, the fourth being still very small at the commencement of the third day.
(2122). At this period three fissures are perceptible between the branchial arches, and in front of the first pair is the first appearance of the oral orifice, - which, however, is not, properly speaking, the aperture of the mouth, since at this epoch the jaws and buccal cavity are not as yet formed, but, physiologically considered, it rather represents the pharynx.
(2123). At the close of the third day this branchial apparatus is already slightly changed: the branchial fissures are wider, and the fourth vascular arch is become nearly as large as the others. On the fourth day the first vascular arch is almost imperceptible, and that for two reasons: in the first place, it becomes covered up with cellular tissue; and secondly, it is so much diminished in size towards the second half of the fourth day, that it merely gives passage to a most slender stream of nearly colourless blood. By the close of the fourth day it is no longer recognizable, but, before its disappearance, it is seen to have given off from its most convex point a vessel, which becomes the carotid artery; so that, when the arch itself is atrophied, that portion of it which was connected with the bulb of the aorta becomes the trunk of the carotid.
(2124). The second arch then becomes diminished in size, insomuch that the third and fourth receive the greater part of the blood, while in the meantime a fifth arch makes its appearance behind the fourth; so that in this way there are still four permeable arches.
(2125). While these changes are going on in the vascular canals, the first branchial fissure gradually closes; and to make up for this, a new one is formed between the arch which originally was the fourth and that last developed.
(2126). At the commencement of the fifth day there are consequently again four vascular arches and three branchial fissures on each side; but not the same as those of the third day, since one arch and one fissure have disappeared, and have been replaced by similar parts. During the fifth day, the vascular arch which at first was the second is obliterated, and the two succeeding ones become increased in size; but at the end of the fifth day all the branchial fissures are effaced, being filled up with cellular tissue, and no trace of them is left. The remainder of the metamorphosis seems to depend principally upon changes that occur in the bulbus arteriosus (fig. 373, b), which is by degrees converted into the bulb of the aorta. This part of the arterial system, from being a single cavity, about the fifth day divides into two canals, which become gradually more and more separated and bent upon themselves. The separation of the bulbus arteriosus into two vessels is, in the opinion of Professor Baer, owing to the circumstance that the ventricles gradually become separated by a septum, which, as it grows more complete, causes two distinct currents of blood to be propelled from the heart. The current coming from the right ventricle arrives sooner than the other at the vascular arches, and rushes through the two posterior and through the middle arch of the left side, while the gush of blood from the left ventricle fills the two anterior arches and the middle arch of the right side, - a circumstance depending on the course impressed upon the currents derived from the two ventricles. Each current becomes more and more distinct; and at last each is provided with a proper channel, forming the trunks of the future pulmonary artery and of the future aorta.