330 EMBRYOLOGY. 



and distinctly evolved ; the muscular parts are very apparent, 

 and separated into bundles under the skin ; the nerves are 

 more conspicuous, and the motions of the embryo are stronger; 

 the neck lengthens greatly. In the heart the external separa- 

 tion of the bulbus aortae into two distinct canals follows ; the 

 vessel proceeding from the left ventricle gives off larger 

 carotids from its anterior arches ; on these appear the little 

 thyroid bodies. These two aortal arches (trunci anonymi) 

 represent the earlier third branchial vascular arch ; the 

 asymmetrical vascular arches appearing behind them, on 

 the right side, is the future aorta descendens. From the stem 

 arising out of the right ventricle proceed the two most poste- 

 terior (the earlier fifth) of the branchial vascular arches ; 

 they do not yet give off any pulmonary branches, and still 

 terminate posteriority in the aorta ; at a later period they be- 

 come the proper pulmonary arteries. The corpora Wolffiana 

 become shorter, and smaller every way, and their excretory 

 duct longer ; the kidneys increase in size. The germ-pre- 

 paring sexual organs begin about this time to differ manifestly 

 in their form : the testicles become elongated, cylindrical, and 

 continue of equal size ; the ovaries remain flattened, grow un- 

 equally, the right first ceasing to make any progress and then 

 disappearing, the left enlarging proportionally with the other 

 parts. The oviducts are distinct, but the right, like the ovary 

 to which it corresponds, is arrested in its development. The 

 gall-bladder becomes conspicuous as a diverticulum of the 

 biliary duct. The bursa Fabricii emerges from the cloaca; 

 the allantois grows still more over the embryo. The vessels 

 on the vitellary membrane, especially on its under-surface, 

 are numerous and large ; the veins are turgid and tortuous 

 (fig. 352, A, c), and appear stained of a yellow colour, 

 whence they are often called vasa lutea. 



[ 496. It is in the course of the last days of the second 

 week that the epidermic formations are produced the feather 

 bulbs, the nails, and the scaly coverings of the feet ; ossifi- 

 cation also begins in many bones, the muscular parts get 

 stronger, the eyelids are well formed, and in the ear the tym- 

 panum has appeared. The Wolffian bodies are ever shorter 

 and smaller ; the testes acquire their excretory ducts ; the left 

 ovary is conspicuous, and the corresponding oviduct is hollow, 

 whilst the same parts on the right side have shrunk entirely. 



