280 EMBRYOLOGY 



grown out and show indications of digits. In general the development of the 

 leg is slower than that of the arm. This is an expression of a general rule 

 that the anterior structures develop more rapidly than posterior structures. 

 A prominent tail, containing a neural tube and notochord, reaches its maxi- 

 mum development about this time, attaining a length about one sixth that 

 of the embryo. Subsequently the tail regresses, but in very rare cases it may 

 persist and grow, reaching a length of three inches in the fetus at term. The 

 upper and lower jaws are recognizable, and the external ear is forming. 



In the brain five primary divisions may now be distinguished, with the 

 telencephalon and the diencephalon both very large. In the eye the nervous 

 layer begins its differentiation, becoming very thick and containing several 

 layers of cells. The outer layer of the optic cup remains thin, and a black 

 pigment is deposited in these cells. The internal ear undergoes changes in 

 form, and the endolymphatic duct develops. The olfactory placodes form 

 olfactory pits, which become olfactory sacs. At this time the endodermal 

 pouches begin their differentiation into thymus, parathyroids, and ultimo- 

 branchial bodies. The thyroid gland becomes first bilobed and then solid in 

 structure. In the mouth the three primordia of the tongue fuse. 



Onset of muscular contraction 



At eight weeks (Fig. 188) the embryo is 24 mm. in length and weighs 

 1 g. In addition to an increase in the complexity of structure between six and 

 eight weeks of development, we now find that function of organs begins. Of 

 course, the heart has been functioning steadily since the initiation of the 

 heart beat at about three weeks. During the seventh week the muscles of the 

 body have differentiated and are capable of contraction. Direct electrical 

 stimulation applied to the muscles of eight-week embryos will cause a quick 

 contraction of muscle. Since in most other mammalian embryos tested, re- 

 flexes begin about one day after the first responses to direct stimulation of 

 muscle, it is safe to conclude that at about eight weeks the human embryo 

 shows simple reflex movements. For example, cat embryos corresponding in 

 structure to a seven-week human embryo will respond reflexly to mechanical 

 stimuli. If the amnion is tapped firmly, the forelimb will jerk. Touching the 

 forelimb directly also results in a quick twitch, as does electrical stimulation. 

 The neck and head muscles contract reflexly when the snout is stimulated. 

 That these are true reflexes and not the results of direct stimulation of 



