284 EMBRYOLOGY 



the placenta into the maternal blood stream. There is a barrier to excretion 

 in the fetal kidney. A layer of epithelium surrounding the glomerulus pre- 

 vents rapid filtration until at birth this layer breaks and the blood vessels 

 float freely in the capsule. 



Respiratory movements of the chest and abdomen are observed in a 12- 

 week fetus removed from the uterus. These movements are described as 

 shallow breathing, and in a somewhat older fetus are rhythmical and occur 

 about once a second. Breathing of the fetus can be stimulated by the lack of 

 oxygen that occurs when the placenta is detached from the uterus. This ob- 

 servation shows that the respiratory center in the brain is active. 



A question arises regarding the possible intake of amniotic fluid into the 

 fetal lung as a result of these fetal respiratory movements. Experiments gen- 

 erally show that no amniotic fluid gets through the glottis. Injection of 

 colloidal thorium dioxide, which throws a shadow in X-ray photographs, into 

 the amniotic fluid of the human fetus shows definitely that while the thorium 

 is present in the stomach and intestine there is none in the trachea or lungs. 

 However, if breathing becomes more intense because of stimulation by lack 

 of oxygen, some amniotic fluid gets into the lungs. 



The development of reflexes 



The fetus at 16 weeks is 112 mm. long and weighs 105 g. (Fig. 189) - 

 During the period from 12 to 16 weeks the weight of the fetus is increased 

 by a factor of 7. In the previous period of 8 to 12 weeks the factor was 14. 

 It is clear that the rate of increase in weight becomes smaller and smaller as 

 the fetus gets older. For example, from 16 to 20 weeks the fetus only triples 

 its weight. 



An over-all inspection of the fetus shows it to be better proportioned 

 than at 12 weeks. The size of the body has increased more rapidly than the 

 head, but the head is still disproportionately large. 



The fetus now shows natural reflex movements within the amniotic 

 cavity and with the placenta intact. These movements can be detected as early 

 as 14 weeks by the stethoscope but are not usually felt by the mother until 

 about 17 weeks. It will be recalled that stimuli will elicit muscle responses 

 much earlier and therefore the relative quiescence of the fetus must be due 

 to the fact that there are few sources of stimuli. This is understandable, since 

 the embryo floating within the amniotic fluid is relatively isolated from 



