DEVELOPMENT OF THE HEAD. 369 



In this stage of development, the extremities, or limbs, 

 are still entirely wanting ; there is as yet no trace either of 

 arms or legs. The head end, however, has already become 

 markedly distinct or differentiated from the tail end ; more- 

 over, the first rudiments of the brain-bladders appear in 

 front, and the heart appears more or less distinctly on the 

 anterior intestine. A real face is, however, not yet formed. 

 We may also search in vain for any character distinguishing 

 the human embryo, in this stage, from that of other Mammals. 

 (Cf. Fig. M I, B I., a I., and H I. on Plate VII.) 108 



Another week later, at the end of the fourth week, 

 between the twenty-eighth and the thirtieth day of develop- 

 ment, the human embryo is four or five lines in length, or 

 about one centimetre (Fig. 122, IV. , Plate VII. Fig. M II.). 

 The head with its various parts is now plainly distinguish- 

 able : within, the five primitive brain-bladders (fore-brain, 

 mid-brain, twixt-brain, hind-brain, and after-brain) ; at the 

 lower end of the head, the gill-arches, which divide the 

 gill-openings ; on the sides of the head the rudiments of 

 the eyes, two indentations of the outer skin, towards which 

 grow two simple bladders from the side- wall of the fore- 

 brain. Far behind the eyes, above the last gill-arch, the 

 bladder-like rudiment of the organ of hearing is visible. 

 The head, which is very large, is attached to the trunk at 

 a very considerable angle, almost a right angle. The trunk 

 itself is still attached at the centre of its ventral side to the 

 intestinal germ- vesicle ; but the embryo is already still 

 further separated from the latter, which, therefore, protrudes 

 and forms the yelk-sac. Like the front part, the hind part 

 of the body is very much curved, so that the pointed tail 

 end is turned towards the head. The head rests, face down- 



