148 THE FOURTH DAY. [CHAP. 



surface of the cloaca by a long hollow stalk, which places its 

 cavity in communication 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 caecal 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 lining of hypoblast, and communicating 

 with the cloaca by a narrow tube of the same construction. 



Keichert (Entwicklungsgeschichte, s. 186) on the other hand stated that the 

 allantois was formed of two solid 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 Remak (Entwicklung, 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 OP AN EMBRYO CHICK AT THE 



COMMENCEMENT OF THE THIRD DAY (Dobrjnin). 



t. the tail, m. the axial mesoblast of the body, about to form the protovertebras. 

 x f . the roof of x". the neural canal. Dd. the hind end of the hind-gut. SO. 

 somatopleure. Spl. splanchnopleure. u. the mesoblast of the splancbno- 

 pleure carrying the vessels of the yolk-sac, pp. pleuroperitoneal cavity. 

 Df. the epithelium lining the pleuroperitoneal cavity. All. the commencing 

 allantois. w. and y. the hypoblast thickened and projecting on either side of 

 the opening of the allantois. 



