288 EMBRYOLOGY 



secretes growth-promoting factor and gonadotropic hormones. The thyroid 

 contains iodine at six months, and probably much earlier. Of especial interest 

 is the development of the suprarenal gland. The cortex and medulla join at 

 about seven weeks, and the cortex becomes very large in proportion to the 

 size of the fetus as compared with the adult. Indeed the adrenal gland reaches 

 a maximum size at birth and then decreases in size, reaching about one half 

 its birth weight after about three months. The loss in weight is correlated 

 with the partial disappearance of the so-called fetal zone in the cortex of the 

 gland. This fetal zone is presumed to secrete an androgen, and it has been 

 suggested that this androgen in the fetus neutralizes any possible effects of 

 the estrogens of the mother. The medulla of the suprarenal has a common 

 origin with the sympathetic ganglia and secretes adrenaline very early in the 

 fetus of some mammals. Secretion is reported as being very low in the human 

 fetus. 



The sense organs of the advanced fetus are well developed morpho- 

 logically, and there is reason to believe that they are able to receive stimuli. 

 Reflexes in response to light are obtained as early as seven months. The eye- 

 lids, which were formed open and subsequently closed, reopen about this 

 time. The ear may be receptive before birth, according to the results of some 

 experiments where sound produced movements of the fetus. On the other 

 hand, newborn babies do not hear very well. This is thought to be in part 

 due to the fact that amniotic fluid is present in the external ear chamber. 

 Since premature babies hear shortly after birth, it is probable that the fetus 

 can also hear. Taste and smell responses also appear to develop before birth, 

 but experiments are very difficult. In this connection it will be remembered 

 that there is an increased intake of amniotic fluid when saccharine is injected. 

 Furthermore, since premature babies show indications of these senses, it is 

 reasoned that the fetus must also taste and smell. 



During late pregnancy there is a shift in the site of formation of blood 

 in the embryo. Whereas in early embryonic stages the red blood cells arise 

 mostly from the blood islands and the liver, in the fetus the bone marrow 

 begins to produce at about 12 weeks and the spleen a few weeks later. With 

 these two new sources of red cells the production of blood by the liver 

 gradually decreases during late pregnancy. 



At about seven and one-half months the fetus may be born and survive 

 and thus must be considered as fully formed, except possibly for temperature 

 regulation. At the normal term, 266 days from fertilization, the newborn 



