Fig. 358. Transverse section through the head of a Rabbit of the same age as fig. 144 B. (From Kölliker.)
B is a more highly magnified representation of part of A.
rf. medullary groove; mp. medullary plate; rw. medullary fold; h. epiblast; dd. hypoblast; dd´. notochordal thickening of hypoblast; sp. undivided mesoblast; hp. somatic mesoblast; dfp. splanchnic mesoblast; ph. pericardial section of body cavity; ahh. muscular wall of heart; ihh. epithelioid layer of heart; mes. lateral undivided mesoblast; sw. part of the hypoblast which will form the ventral wall of the pharynx.
The above description applies only to the development of the heart in those types in which it is formed at a period after the throat has become a closed tube (Elasmobranchii, Amphibia, Cyclostomata, Ganoids (?)). In a number of other cases, in which the heart is formed before the conversion of the throat into a closed tube, of which the most notable is that of Mammals (Hensen, Götte, Kölliker), the heart arises as two independent tubes ([fig. 358]), which eventually coalesce into an unpaired structure.
In Mammals the two tubes out of which the heart is formed appear at the sides of the cephalic plates, opposite the region of the mid- and hind-brain ([fig. 358]). They arise at a time when the lateral folds which form the ventral wall of the throat are only just becoming visible. Each half of the heart originates in the same way as the whole heart in Elasmobranchii, etc.; and the layer of the splanchnic mesoblast, which forms the muscular wall for each part (ahh), has at first the form of a half tube open below to the hypoblast.
Fig. 359. Two diagrammatic sections through the region of the hind-brain of an embryo Chick of about 36 hours illustrating the formation of the heart.
hb. hind-brain; nc. notochord; E. epiblast; so. somatopleure; sp. splanchnopleure; d. alimentary tract; hy. hypoblast; hz. heart; of. vitelline veins.
On the formation of the lateral folds of the splanchnic walls, the two halves of the heart become carried inwards and downwards, and eventually meet on the ventral side of the throat. For a short time they here remain distinct, but soon coalesce into a single tube.
In Birds, as in Mammals, the heart makes its appearance as two tubes, but arises at a period when the formation of the throat is very much more advanced than in the case of Mammals. The heart arises immediately behind the point up to which the ventral wall of the throat is established and thus has at first a Lambda-shaped form. At the apex of the Lambda, which forms the anterior end of the heart, the two halves are in contact ([fig. 357]), though they have not coalesced; while behind they diverge to be continued as the vitelline veins. As the folding in of the throat is continued backwards the two limbs of the heart are brought together and soon coalesce from before backwards into a single structure. [Fig. 359] A and B shews the heart during this process. The two halves have coalesced anteriorly (A) but are still widely separated behind (B). In Teleostei the heart is formed as in Birds and Mammals by the coalescence of two tubes, and it arises before the formation of the throat.
The fact that the heart arises in so many instances as a double tube might lead to the supposition that the ancestral Vertebrate had two tubes in the place of the present unpaired heart.
The following considerations appear to me to prove that this conclusion cannot be accepted. If the folding in of the splanchnopleure to form the throat were deferred relatively to the formation of the heart, it is clear that a modification in the development of the heart would occur, in that the two halves of the heart would necessarily be formed widely apart, and only eventually united on the folding in of the wall of the throat. It is therefore possible to explain the double formation of the heart without having recourse to the above hypothesis of an ancestral Vertebrate with two hearts. If the explanation just suggested is the true one the heart should only be formed as two tubes when it arises prior to the formation of the throat, and as a single tube when formed after the formation of the throat. Since this is invariably found to be so, it may be safely concluded that the formation of the heart as two cavities is a secondary mode of development, which has been brought about by variations in the period of the closing in of the wall of the throat.