OF ARTS AND SCIENCES. 13 



" From each of these tubes there then project short digitations, 

 which are to be the future uriniferous tubes of this organ ; the original 

 tube becoming the duct of them all in each organ. 



" These ducts pass down to the last caudal vertebrse, over which 

 they turn and come together; at their point of junction appears a small 

 vesicle, the expansion of their combined extremities. This vesicle — 

 a minute sphere, and scarcely to be distinguished from the extremities 

 of the ducts themselves — is the allantois in its earliest condition. 



" At first its walls are extremely thin, being of a most delicate 

 merribrane ; but as its size increases, cells appear upon its inner sur- 

 face, and at last a basement membrane is perceived, covered with 

 epithelial cells. All these formative changes have taken place be- 

 neath the investing membrane of the whole embryo, and directly at 

 the point of the branching of the two umbilical arteries. 



" As the vesicle expands, it pushes out, first, the branches of these 

 arteries which rest upon it, and by anastomosis form a network ; sec- 

 ond, a hood of the membrane investing the whole embryo. In less 

 than a day after this, when the vesicle has attained the diameter of one 

 sixteenth of an inch, the network of vessels united in the hood of the 

 investing membrane has so increased, that it seems to form the vesicle 

 proper, the original membrane being entirely masked. At this period 

 the allantois has very much the aspect of a diverticulum of the in- 

 vesting membrane of the embryo, and to this perhaps is due the opin- 

 ion of Coste as to its origin. 



" After this it increases rapidly, the spherical vesicle becoming 

 flask-shaped, and extending out quite beyond the caudal vertebrae, 

 around which it passes to reach the dorsal surface of the embryo. 

 Here it meets the amnion, with the membranes of which it partly 

 blends, and in this way serves to conduct to it the umbilical vessels. 



" Such is its mode of formation. Its functional relations are equal- 

 ly interesting, 



" I would remark, in the first place, that the Wolffian bodies are 

 truly depurating organs of the blood ; in fact, are the temporary kid- 

 neys of the embryo. We have seen that the allantois appears as the 

 bulbous termination of their combined ducts, at a very early period of 

 embryonic life. But it does not arise until the Wolffian bodies have 

 attained a functional power ; that is, until uriniferous tubes are formed 

 having direct relations with the bloodvessels. Indeed, the allantois, 

 as the receptacular termination of the ducts of the Wolffian bodies, is 

 not formed until a urinary secretion is produced. 



