614 OF REPRODUCTION. 



Yolk, and of conversion into Blood, probably continues as long as there is any 

 alimentary material left in the sac. The Yolk-sac is early separated in the 

 Mammalia, by a constriction of the portion which is continuous with the abdo- 

 men of the Embryo ; and it is known from that time under the name of the 

 Umbilical Vesicle. The communication, however, remains open for a time 

 through the constricted portion, which is termed the Vitelline Duct ; and even 

 after this has been cut off, the trunks which connected the circulating system 

 of the Embryo with that of the Vascular Area, are still discernible ; these are 

 called Omphalo-Mesenteric, Meseraic, or Vitelline vessels. It was formerly 

 believed, that the nutrient matter of the yolk passes directly through the Vitel- 

 line duct, into the (future) digestive cavity of the Embryo, and is from it 

 absorbed into its structure ; but there can now be little doubt that the Vitelline 

 vessels are the real agents of its absorption, and that they convey it to the tis- 

 sues in process of formation. They do, in fact, correspond to the Mesenteric 

 veins of Invertebrated animals, which are the sole agents in the absorption of 

 nutriment from their digestive cavity ( 462) ; and the yolk-bag, as already 

 remarked, is the temporary stomach of the Embryo, remaining as the per- 

 manent stomach in the Radiated tribes. Previous to the ninth day of incuba- 

 tion (in the Fowl's egg), a series of folds are formed by the lining membrane 

 of the yolk-bag, which project into its cavity ; these become gradually deeper 

 and more crowded, as the bag diminishes in size by the absorption of its con- 

 *tents. The Vitelline vessels, that ramify upon the yolk-bag, send into these 

 folds (or valvulae conniventes) a series of inosculating loops, which immensely 

 increase the extent of this absorbing apparatus. But these minute vessels are 

 not in immediate contact with the yolk ; for there intervenes between them a 

 layer of nucleated cells, which is easily washed away. It was from the colour 

 of these, communicated to the vessels beneath, that Haller termed the latter 

 vasa lutea; when the layer is removed, the vessels present their usual colour. 

 There seems good reason to believe that these cells, like those of the Intesti- 

 nal Villi in the adult ( 461), are the real agents in the process of absorbing 

 and assimilating the nutritive matter of the yolk; and that they deliver this 

 up to the vessels, by themselves undergoing rupture or dissolution, being 

 replaced by new layers. 



763. The formation of the Heart takes place in the Vascular layer, beneath 

 the upper part of the Spinal Column ; it at first appears as a mere cavity in 

 its substance, surrounded only by cells ; but its walls gradually acquire firm- 

 ness and distinctness, and become sufficiently powerful to propel the bk>od 

 through the vessels of the Embryo and those of the Vascular Area. The first 

 appearance of the Heart in the Chick is at about the 27th hour ; the time of its 

 formation in Mammalia has not been distinctly ascertained. In its earliest 

 form, it has the same simple character which is presented by the central im- 

 pelling cavity of the lower Invertebrata ; being a mere prolonged canal, which 

 at its posterior extremity receives the veins, and at its anterior sends forth the 

 arteries. After a short time, however, it becomes bent upon itself (Plate II., 

 Fig. 27, d) ; and it is soon subdivided into three cavities, which exist in all 

 Vertebrata, a simple auricle or receiving cavity, a simple ventricle or pro- 

 pelling cavity, and a bulbus arteriosus at the origin of the aorta. The cir- 

 culation is at first carried on exactly upon the plan which is permanently 

 exhibited by Fishes. The aorta subdivides into four or five arches on either 

 side of the neck ; and these are separated by slits or fissures, much resembling 

 those which form the entrances to the gill-cavities of Cartilaginous Fishes. 

 These arches reunite to form the descending aorta, which transmits branches 

 to all parts of the body. Such is the first phase or aspect of the Circulating 



* Grainger and Dalrymple, in Baly's translation of Muller's Physiology, pp. 1557-1560 



