THE HUMAN FETUS AT TWELVE WEEKS 283 



The human fetus at twelve weeks 



At 12 weeks the fetus is 56 mm. long and weighs 14 g. That is, the 

 fetus now weighs 14 times as much as it did at eight weeks and is about two 

 and one-half times as long. It begins to take on a more human appearance, 

 although the head is relatively too large. The face is much better developed, 

 with the lips formed, the nose protruding, and the ears acquiring a more 

 adult shape. The head and neck begin to straighten up somewhat. The eye- 

 lids are fused at this time. Reflex movements of the lips suggesting the first 

 beginnings of the sucking reflex have been observed as early as ten weeks. 

 These lip movements become more complicated with time. 



The limbs are relatively longer and slenderer, the fingers are very well dif- 

 ferentiated, and the nails are beginning to form. The muscles and nerves of 

 the arm are well developed, as is evidenced by reflex stimulation of the fingers. 

 Movements of the fingers after stimulation by touch has been recorded as early 

 as 1 1 weeks. The fingers curl toward the palm. At 12 weeks the fetus can make 

 a fist. Some weeks later the grasp reflex operates, and the fingers will hold an 

 object tightly. Similarly the foot responds to the stimulation of touch of the 

 sole at 12 weeks or even less. The big toe curls upward and the rest of the 

 toes spread; this is called the Babinski reflex. These reflexes involve only 

 the spinal cord and not the higher brain centers. A sensory mechanism must 

 be operating, since tapping on the chorion at this stage causes contraction of 

 the arms and legs. 



Other organs begin to function about this time. Experiments on the rabbit 

 and the cat fetus in which dyes are injected into the body show that the 

 metanephros as well as the mesonephros function very early. From this 

 evidence it is estimated that the permanent kidney of the human fetus begins 

 to function at nine weeks, at a time when the mesonephros is still active. Since 

 the mesonephros is fully differentiated at seven weeks and the metanephros 

 differentiates its first tubules at this time, it is clear that by nine weeks the 

 metanephros may be actively excreting. Urea and uric acid are both excreted 

 by the kidney, and the excretory products form part of the amniotic fluid, 

 which increases from 40 ml. at 10 weeks to 1800 ml. at 38 weeks. During this 

 period the concentration of urea in the fluid changes from 35 to 45 mg. per 

 100 ml. and that of uric acid from 3.5 to 5.1 mg. per 100 ml. Excretion 

 through the kidney, however, can be only slight compared to excretion across 



