HIRDS. 855 



off four vascular arches on each side, and between these tln-ee 

 fissures are formed. This disposition, which reminds us of the 

 class of fishes, was by Rathke, who first discovered it, actually 

 compared with the structure in that class; hence the names of 

 branchial fissures and branchial arches \ On the fourth day a fifth 

 pair of branchial arches is formed, and the first arch ceases to 

 convey blood; the current also in the second becomes weaker, and 

 this arch disappears on the fifth day, when thus only three pairs of 

 vascular arches remain. These four or three arches form on each 

 side a short stem; the two stems unite as roots of the large artery, 

 to form a single main stem. The large artery thus formed divides, 

 however, again lower down into two branches, and from these the 

 arteries proceed, which are distributed transversely over the ger- 

 minal membrane. In this period also rudiments of the liver, the 

 pancreas, the allantois and the lungs are formed. All these parts, 

 according to VoN Baer, are merely excrescences from the intestinal 

 canal, of which the mucous membrane penetrates the vascular layer 

 and thus forms these parts as conical, hollow appendages. On each 

 side of the inferior surface of the spinal column the vascular layer 

 forms a lamina, whilst the mucous layer expands downwards; from 

 the coalescence of these two laminse the mesentery arises. In this 

 period also the Wolffian bodies or primordial kidneys arise, which 

 are situated on each side of the back along the greatest part of the 

 trunk, consisting of many tortuous, transverse, blind tubes and 

 having an excretory duct which opens into the cloaca^. The first 

 rudiments of limbs are seen, on the fourth day, as small excrescences 

 on the ventral laminas; on the fifth day they are larger; at that 

 time they end in a flat and round part which is attached to a nar- 

 row pedicle. 



The circulation of blood through the allantois, which may 

 be regarded as a respiratory organ, supersedes more and more 

 after the sixth day the circulation through the vasa om^halo- 

 meseraica, and characterises the last period, that, namely, from 

 the sixth day to the end of brooding. The allantois, of which the 



^ Reichekt names them visceral arches. See his Dissertatio de Emhryonum arcuhus 

 sic dictis hranckialihus, Berolini, 1836, 4to. 



2 Compare on these parts, amongst others, Rathke Beitr. zur Gesch. der Thierwelt, 

 III. 1825, s. 50 f. f. ; J. Mueller £Udungsgesch. der Genitalien, Dusseldorf, 1830, 4to, 

 s. 11 f. f. &c. 



23—2 



