734 DEVELOPMENT 



these, after aeration, to the left auricle and ventricle, to be distributed over 

 the body. 



Parturition. With the implantation of the embryo and the develop- 

 ment of the placenta, the uterus grows rapidly until the end of pregnancy. 

 The muscles of its walls increase enormously in volume, apparently by 

 an increase, in the size of the fibers, and the whole structure may become 

 thirty or forty times its size in the resting period. Many changes take 

 place also in other parts of the body, changes which are dependent on the 

 presence of the fetus. Full-term pregnancy occurs when the uterus is 

 isolated from the nervous system, hence it has been inferred that there is 

 some sort of special secretion, possibly of the embryo itself, that makes its 

 way into the blood and influences the organs of the mother. 



FIG. 506. FIG. 507. 



FIG. 506. Section of Mammary Gland of Bitch, Showing Acini, Lined with Fpithelial 



Cells of a Polyhedral or Short Columnar Form. X 200. (V. D. Harris.) 

 FIG. 507. Globules and Molecules of Cow's Milk. X 400. 



At the end of the period of pregnancy the strong uterine walls begin 

 periodic contractions which ultimately result in the delivery of the fetus. 

 These contractions are at first weak and at long intervals, but later become 

 very strong and follow each other in rapid succession. The uterine con- 

 tractions are supported by reflex contractions of the abdominal and thoracic 

 muscles. After the fetus is delivered the uterine contractions become 

 milder, but still continue until the placenta is finally expelled. 



The initiation of the contractions of the uterus at delivery probably de- 

 pends on the chemical stimulation of some substance or substances produced 

 in the uterus itself or in the fetus; substances that react on the nervous mech- 

 anism and on the uterine muscles themselves. This view cannot be said to 

 be proven, but it is supported by certain observed facts and experiments. 



Lactation. There is a marked development of the mammary glands, 

 especially in the later part of the period of gestation. Upon delivery of the 

 fetus the gland enlarges very sharply and an abundant secretion is formed. 



