REPRODUCTION 755 



ulates on boiling, and certain inorganic salts which have a laxative effect 

 on the new-born child. Normal lactation and the phenomena which 

 accompany it are fully established by the end of the second or third day. 



The composition of milk and the mechanism of its production have been 

 stated in the chapter on Secretion. 



Physiologic Activities of the Embryo. During intrauterine life the 

 evolution of structure is accompanied by an evolution of function. The 

 relatively simple and uniform metabolism of the undifferentiated blastoder- 

 mic membranes gradually increases in complexity and variety, as the individ- 

 ual 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 functionate,, 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 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 eliminated by the placental structures; 

 but as metabolism increases in complexity the liver and kidney assume ex- 

 cretory 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, in- 

 dicating the establishment of both hepatic and renal activity. Contractions 

 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, automatic 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 subserve general 

 metabolism begin to functionate and in the course of a week are fully estab- 

 lished. 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 propor- 

 tional to the food consumed; the digestive glands are elaborating their re- 

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

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

 activites 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. 



