The elements of Embryology, (1874) The elements of Embryology, elementsofembryo74fost Year: 1874 148 THE FOURTH DAT. [chap. siirface of the cloaca by a long hollow stalk, which places its cavity in commiinication with that of the alimentary canal. Both vesicle and stalk have an outer coat of mesoblast and an inner lining of what apparently is hypoblast. So much any observer may readily determine for himself; but of the earliest stages of the development of this organ different embryologists have given very different accounts. Von Baer believed that, soon after the cloaca was formed by the enl
The elements of Embryology, (1874) The elements of Embryology, elementsofembryo74fost Year: 1874 148 THE FOURTH DAT. [chap. siirface of the cloaca by a long hollow stalk, which places its cavity in commiinication with that of the alimentary canal. Both vesicle and stalk have an outer coat of mesoblast and an inner lining of what apparently is hypoblast. So much any observer may readily determine for himself; but of the earliest stages of the development of this organ different embryologists have given very different accounts. Von Baer believed that, soon after the cloaca was formed by the enlargement of the csecal hind end of the alimentary canal, the allantois arose from it as a spherical diverticulum, generally visible about the middle of the third day, in whose formation both of the coats of the alimentary canal took part. This spherical diverticulum gradually lengthened out into a pear-shaped vesicle, connected with the cloaca by a hollow stalk which rapidly narrowed and lengthened, until the allantois formed an independent hollow body, composed of an outer coat of mesoblast and a Uning of hypoblast, and communicating with the cloaca by a narrow tube of the same construction. Eeichert {EntwicHungsgeschichte, a. i86) on the other hand stated that the allantois was formed of two soHd outgrowths from the mesoblast of the somato- pleure, which subsequently coalesced and became hollow; but believed that it was primarily connected with the Wolffian ducts and not with the cloaca. According to Eemak {EntwicHung, § 57, 58) it is formed by two solid vascular outgrowths of the mesoblast of the body-wall, one on each side of the middle line, which project in the pleuroperitoneal cavity near to the cloaca. These two outgrowths coalesce, and then grow up, till they come in contact with Fig. 49. Longitudinal Section of the Tail-end of an Embeto Chick at the COMMENCEMENT OF THE THIED DaT (Dobrjnin). t. the tail, m. the axial mesoblast of the body, about to form the protove
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