286 EMBRYOLOGY 



of saccharine into the amniotic fluid to sweeten it, the fetus was induced to 

 swallow more amniotic fluid and actually reduce the volume. Of course, this 

 reduction in volume must be due to absorption of the water by the fetal 

 digestive tract and subsequent elimination through the placenta and the urine 

 of the mother. Indeed, methylene blue injected into the amniotic fluid appears 

 in the urine of the mother. 



Other parts of the digestive system become functional about this time. 

 The oral glands contain ptyalin; the stomach glands contain pepsin; the 

 liver secretes bile, which flows into the intestine and collects there; and 

 trypsin is said to be secreted at about the fifth month. In the pancreas the 

 islands of Langerhans have formed and may be secreting insulin. Insulin 

 is secreted by the dog fetus in amounts such that when the pancreas of a 

 pregnant dog is removed no excess sugar appears in its urine. The insulin 

 from the fetus stimulates enough carbohydrate metabolism to keep the blood 

 sugar of the mother low. The hormone secretin has been demonstrated from 

 the small intestine at about 18 weeks. Thus, it seems clear that the digestive 

 system is well on its way toward becoming functional both mechanically and 

 chemically between the sixteenth and twentieth weeks of development. It is 

 interesting to point out that at this time the fetus weighs only a few ounces — 

 about 3.5 ounces at 16 weeks and about 10 ounces at 20 weeks. 



At 16 weeks most of the bones of the body are clearly formed, although 

 ossification is by no means complete and continues long after birth. This 

 period is characterized by a degeneration of the mesonephros and the almost 

 complete differentiation and function of the metanephros. 



Development from 20 to 38 weeks 



During the remaining weeks (20 to 38) growth continues to slow up. 

 From 20 to 24 weeks the fetus nearly doubles its weight, whereas from 24 

 to 28 weeks the factor is 1.7. From 28 to 32 weeks the factor drops to 1.5 

 and for the 32-to-36-week period it falls to 1.4. This slowing of the growth 

 rate is probably accompanied by a decrease in the rate of metabolism. During 

 this period there is also observed a decrease in the rate of heart beat. The 

 heart of a five-month fetus beats at more than twice the rate of the adult — 

 160 per minute as compared with 70. The heart rate decreases to about 140 

 at full term. During birth and the first few minutes after birth the heart beat 

 undergoes regular changes. The rate slows during birth, and this may be an 



