PHYSIOLOGY OF THE EMBRYO 321 



permanent kidneys are formed by the diverticuli of the 

 posterior end of the Wolffian duct. These diverticuli branch 

 and the branches become the uriniferous tubules of the kid- 

 neys ; at their closed ends a glomerulus forms. In the 

 female the Wolffian duct is obliterated, while in the male it 

 forms the vas deferens. 



3. The sebaceous secretion begins in the fifth month. 

 The substance thus secreted forms a fatty layer upon the 

 skin and is called the vernix caseosa. 



The removal of metabolic waste products from the embryo 

 is accomplished not only by the glands (liver and kidneys), 

 but also by the exchange of gases between the fcetal and 

 the maternal blood in the placenta. 



(e) Metabolism. The metabolism of the embryo is small ; 

 little heat needs to be produced, for the loss of heat is ex- 

 ceedingly small. The muscular movements which could 

 increase metabolism are very limited. Hence the food sup- 

 plied to the embryo is chiefly used for its growth. 



3. THE TRANSFORMATION AND SETTING FREE OF 

 ENERGY IN THE EMBRYO 



(<?) Muscular movements. The first appearance of the 

 skeletal muscle is during the second month of pregnancy. 



Muscular movements, excepting the beat of the heart, 

 begin at the fifth or sixth month. They consist of jerky 

 movements of the limbs against the walls of the uterus. 

 The movements of the foetus appear to be reflex movements ; 

 they are increased when the foetus is pushed by pressing upon 

 the abdominal walls of the mother. At the close of preg- 

 nancy, weak rhythmic respiratory movements are sometimes 

 made, also movements of sucking and deglutition; swallowed 

 amniotic fluid may be found in the embryo. 



(b) The development of the functions of the nervous 

 system. The researches concerning the medullation of the 

 nerves furnish the basis for judging the development of these 

 functions. The nerve fibres at first have no medullary 



