THE THIRD WEEK. 



491 



curiously characteristic of the embryo Lg, and of others of the 

 same age. 



The greatest length of the embryo Lr, measured in a straight 

 line, from the swelling caused by the mid-brain to the rounded 

 hinder end of the body, is 4-2 mm. 



In the head, the outlines of the several brain vesicles can be 



Fie. 198. — Human Embryo lettered by Professor His, Lr, and estimated as 

 twenty or twenty-one days old. The right wall of the pericardial cavity 

 has been removed, to expose the heart ; and the arteries, the veins, and 

 the hinder part of the alimentary canal axe represented as though the 

 embryo were transparent. (From His.) x 23. 



A, dorsal aorta. A.l, first aortic arch, in the mandibular arch. A. 2, second aortic 

 arch, in the hyoid arch. A.3, third aortic arch, in the first branchial arch. A.4, fourth 

 aortic arch, in the second branchial arch. A.5, fifth aortic arch, in the third branchial 

 arch. AA, allantoic artery. EI, auditory vesicle. GH, hind-gut. GT, mid-gut, 

 opening into the yolk-stalk. HM, hyomandibular cleft or groove. OF, olfactory pit. 

 RA^ right auricle. RT, truncus arteriosus. RV, ventricle. TA, diverticulum of 

 hind-gut into allantoic stalk. TL, tail. TR, cloaca. TZ, allantoic stalk. TA, 

 allantoic vein. VB, anterior cardinal vein. "VC, posterior cardinal vein. VD, Cu- 

 vierian vein. VV, vitelline vein. TTK, yolk-stalk. 



recognised fairly accurately through the skin ; and the propor- 

 tions have not altered very greatly from those of the earlier 

 embryo, Lg. The axes of the fore-brain and of the hind-brain 

 are still at right angles to each other, the mid-brain forming a 

 prominent rounded swelling at their junction. The fore-brain 

 is wider from side to side than before, owing to the optic vesicles 

 which project outwards from its sides. 



