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

by Francis M. Balfour

The heart.

The first trace of the heart becomes apparent during stage G, as a cavity between the splanchnic mesoblast and the wall of the gut immediately behind the region of the visceral clefts (Pl. 11, fig. 4, ht.).

The body-cavity in the region of the heart is at first double, owing to the two divisions of it not having coalesced; but even in the earliest condition of the heart the layers of splanchnic mesoblast of the two sides have united so as to form a complete wall below. The cavity of the heart is circumscribed by a more or less complete epithelioid (endothelial) layer of flattened cells, connected with the splanchnic wall of the heart by protoplasmic processes. The origin of this lining layer I could not certainly determine, but its connection with the splanchnic mesoblast suggests that it is probably a derivative of this[314]. In front the cavity of the heart is bounded by the approximation of the splanchnic mesoblast to the wall of the throat, and behind by the stalk connecting the alimentary canal with the yolk-sack.

As development proceeds the ventral wall of the heart becomes bent inwards on each side on a level with the wall of the gut (Plate 11, fig. 4), and eventually becomes so folded in as to form for the heart a complete muscular wall of splanchnic mesoblast. The growth inwards of the mesoblast to form the dorsal wall of the heart does not, as might be expected, begin in front and proceed backwards, but commences behind and is gradually carried forwards.

From the above account it is clear that I have failed to find in Elasmobranchii any traces of two distinct cavities coalescing to form the heart, such as have been recently described in Mammals and Birds; and this, as well as the other features of the formation of the heart in Elasmobranchii, are in very close accordance with the careful description given by Götte[315] of the formation of the heart in Bombinator. The divergence which appears to be indicated in the formation of so important an organ as the heart between Pisces and Amphibians on the one hand, and Aves and Mammalia on the other, is certainly startling, and demands a careful scrutiny. The most complete observations on the double formation of the heart in Mammalia have been made by Hensen, Götte and Kölliker. These observations lead to the conclusion (1) that the heart arises as two independent splits between the splanchnic mesoblast and the hypoblast, each with an epithelioid (endothelial) lining. (2) That the heart is first formed at a period when the folding in of the splanchnopleure to form the throat has not commenced, and when therefore it would be impossible for it to be formed as a single tube.

In Birds almost every investigator since von Baer has detected more or less clearly the coalescence of two halves to form the unpaired heart[316]. Most investigators have however believed that there was from the first an unpaired anterior section of the heart, and that only the posterior part was formed by the coalescence of two lateral halves. Professor Darlste His, and more recently Kölliker, have stated that there is no such unpaired anterior section of the heart. My own recent observations confirm their conclusions as to the double formation of the heart, though I find that the heart has from the first a Λ-shaped form. At the apex of the Λ the two limbs are only separated by a median partition and are not continuous with the aortic arches, which do not arise till a later period[317]. In the Bird the heart arises just behind the completed throat, and a double formation of the heart appears, in fact, in all instances to be most distinctly correlated with the non-closure of the throat, a non-closure which it must be noted would render it impossible for the heart to arise otherwise than as a double cavity.

In the instances in which the heart arises as a double cavity it is formed before the complete closure of the throat, and in those in which it arises as a single cavity it is formed subsequently to the complete formation of the throat. There is thus a double coincidence which renders the conclusion almost certain, that the formation of the heart as two cavities is a secondary change which has been brought about by variations in the period of the closing in of the wall of the throat.

If the closing in of the throat were deferred and yet the primitive time of formation of the heart retained, it is clear that such a condition as may be observed in Birds and Mammals must occur, and that the two halves of the heart must be formed widely apart, and only eventually united on the folding in of the wall of the throat. We may then safely conclude that the double formation of the heart has no morphological significance, and does not, as might at first sight be supposed, imply that the ancestral Vertebrate had two tubes in the place of the present unpaired heart. I have spoken of this point at considerable length, on account of the morphological importance which has been attached to the double formation of the heart. But the views above enunciated are not expressed for the first time. In the Elements of Embryology we say, p. 64, “The exact mode of development (of the heart) appears according to our present knowledge to be very different in different cases; and it seems probable that the differences are in fact the result of variations in the mode of formation and time of closure of the alimentary canal.” Götte again in his great work[318] appears to maintain similar views, though I do not perfectly understand all his statements. In my review of Kölliker's Embryology[319] this point is still more distinctly enunciated in the following passage: “The primitive wide separation and complete independence of the two halves of the heart is certainly surprising; but we are inclined, provisionally at least, to regard it as a secondary condition due to the late period at which the closing of the throat takes place in Mammals.”

The general circulation.

The chief points of interest in connection with the general circulation centre round the venous system. The arterial arches present no peculiarities: the dorsal aorta, as in all other Vertebrates, is at first double (Pl. 11, fig. 6, ao), and, generally speaking, the arrangement of the arteries accords with what is already known in other forms. The evolution of the venous system deserves more attention.