The early development of the pericardium, diaphragm, and great veins / by C.B. Lockwood.

  • Lockwood, Charles Barrett, 1856-1914.
Date:
1888
Catalogue details

Licence: Public Domain Mark

Credit: The early development of the pericardium, diaphragm, and great veins / by C.B. Lockwood. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    36G owing to multiplication and elongation of its cells. At this time the portion of the coelom into which the cardiac loops project is more capacious than the rest, but has not the same lateral extension. Its somatopleiire, after running a little way out- wards, turns abruptly ventralwards to join the splanchnopleure. At the foremost end of the rudimentary heart, where the bending of the somatopleure is greatest, both membranes disassociate themselves at an early period from the peripheral uncleft mesoblast; but further back they retain their original connexions. The accompanying drawing was made from a section which was slightly oblique, and illustrates both of these points (fig. 1). Afterwards, as development proceeds, the whole of the cardio- cephalic region ultimately frees itself in this way from the peripheral uncleft meso- blast. The splanchnic cardiac loops grow rapidly during the eighth day, and each acquires an endothelial lining. The latter, as Balfour points out; is derived from the splanchnic mesoblast, and cells of a similar nature reach inwards towards the noto- chord, and are concerned in the formation of the dorsal aortas. The endothelial linings form a continuous ring, although the splanchnic cardiac loops are incomplete ventralwards (fig. 2, Ht). As the pharynx closes in, the separate halves of the heart gradually approximate (fig. 3, Ht.), and finally unite at the ventral asjsect of the alimentary canal. When this is completed the heart remains for a time attached to the pharynx by the mesocardium posterius, and to the ventral wall of the coelom by the mesocardium anterius. From the fore end of the heart the aortae pass round the pharynx to join the dorsal aortae, which, although distinctly formed, have not as yet coalesced. During the first half of the eighth day the aortae are the only vessels which leave or enter the heart, but afterwards the vitelline veins develop and exercise an important influence upon the development of the pericardium. The specimens m my possession (fig. 3) do not seem to favour the view that the cardiac loops become complete tubes before their union'; it seems as if their concavities merely come together and unite; but, of course, the endothelial linings must be excepted from this statement. Moreover, as the halves of the heart join together the continuity of the hypoblast between them is broken, and its epithelium takes no part in the formation of the organ. Hence it follows that the hypoblast which lines the pharynx ceases to unite with that which clothes the ventral aspect of the cardiac coelom by passing through the heart or mesocardia. The effect of these changes upon the cardiac coelom is comparatively simple. As the splanchnopleure closes in to form the pharynx the separate halves of the coelom come together, but are, at first, partitioned from one another by the mesocardium posterius and anterius, with the heart in their midst; and thus a ventral mesentery originates which is attached to the pharynx and supports the heart. By the beginning of the ninth day the mesocardium anterius has disappeared, with the exception of a small remnant which persists for some days, and the halves of the * Balfour, ‘ Elements of Embryology,’ vol. 2, p. 633.