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. ML, JBL, CL, and HI. on Plate VII.) 103 



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- 



