REPRODUCTION. 669 



complexity and variety, as the individual tissues and organs make 

 their appearance and assume even a slight degree of functional 

 activity. As to the periods at which different organs begin to func- 

 tionate, but little is positively known. 



The primitive heart, in all probability, begins to pulsate very 

 early, as in an embryo from fifteen to eighteen days old and measuring 

 but 2.2 mm. in length, Coste found the amnion, the allantois, the 

 omphalo-mesenteric vessels, and the two primitive aortae developed. 

 In the earlier weeks, all products of metabolism are doubtless elimi- 

 nated by the placental structures; but as metabolism increases in 

 complexity the liver and kidney assume excretory activity. Thus, 

 at the end of the third month the intestine contains a dark, greenish, 

 viscid material meconium composed of bile pigments, bile salts, 

 and desquamated epithelium ; the amniotic fluid, as well as the fluid 

 within the bladder, contains urea at the end of the sixth month, 

 indicating the establishment of both hepatic and renal activity. Con- 

 tractions of the skeletal muscles of the limbs begin about the fifth 

 month, from which it may be inferred that the mechanism for muscle 

 activity, viz., muscles, efferent nerves, and spinal centers, has 

 become anatomically developed and associated, and capable of 

 coordinate activity. These contractions are, in all probability, auto- 

 matic or autochthonic in character due to stimuli arising within the 

 spinal centers. The remaining organs remain more or less inactive. 



After birth, with the first inspiration and the introduction of food 

 into the alimentary canal, the physiologic mechanisms which sub- 

 serve general metabolism begin to functionate and in the course 

 of a week are fully established. At this time the cardiac pulsation 

 averages about 135 a minute; the respiratory movements vary from 

 30 to 35 a minute, and are diaphragmatic in type; the urine, which 

 was at first scanty, is now abundant and proportional to the food 

 consumed; the digestive glands are elaborating their respective 

 enzymes, digestion proceeding as in the adult. The hepatic secre- 

 tion is active and the lower bowel is emptied of its contents; the 

 coordinate activities of the nerve-, muscle-, and gland-mechanisms 

 are entirely reflex in character. Psychic activities are in abeyance by 

 reason of the incomplete development of the cerebral mechanisms. 



