MAN 309 



appears on one side of the germ, and later a groove, which 

 is the beginning of the nervous system, forms down its 

 center. The germ layers (ectoderm, entoderm, mesoderm) 

 are soon formed, the enteron and notochord take form, 

 and at the end of the third week the heart of the embryo 

 begins to pulsate (Fig. 115). 



The human embryo has unusual opportunities for de- 

 velopment. It is nourished for nine months in the uterus 

 of its mother from her blood and rests in watery fluids 

 surrounded by protective membranes (Fig. 106, page 288). 

 The fully formed embryo, or fcetus, does not get blood 

 directly from its mother. There is a transfer of nourish- 

 ment and oxygen through thin membranes in the placenta. 

 The blood in the foetal circulation is very poor in oxygen 

 and the fcetus is remarkably resistant to asphyxiation. 

 This is a valuable adaptation for it prevents suffocation at 

 the time of birth when the circulation from placenta to 

 child is often cut off completely for some time. While 

 within the mother the blood of the foetus passes through 

 the foramen ovale, an opening which makes a short cut 

 between the two sides of the heart. There are other short 

 routes so that most of the oxygen in the blood goes to the 

 liver, heart, and head. At birth these channels close and 

 there are fundamental changes in circulation and respira- 

 tion. Sometimes some of them fail to close properly and 

 a "blue baby" or other defective, which usually dies soon, 

 results. 



During the growth of the child the mammary glands of 

 the mother have been changing and soon after birth they 

 begin to secrete milk. This is a true glandular secretion 

 and not merely "filtered blood." A child, then, like other 

 mammals, has unusually advantageous conditions for 

 development in that it may attain an age of nearly two 

 years (after fertilization) without having any nourishment 

 except that supplied by the mother. At about the time 

 of birth important nervous connections are established by 

 the growth of certain tracts of nerves through the brain and 



