930 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



in this way is mixed with aerated blood within the descending aorta. Some 

 circulation to and from the capillary bed within the lungs also occurs at 

 this period. 



At birth and after, the change in the place of oxygenation of the blood 

 from the placental area to the lungs with the stoppage of the blood flow 

 through the umbilical vessels, necessitates the changes shown in figure 379B. 

 The closure, normally, of the foramen ovale in interatrial septum II, together 

 with the shrinkage of the ductus arteriosus to form the ligamentum arteriosum 

 accommodates this change in direction of blood flow. The alterations which 

 effect the stoppage of blood flow through the foramen ovale and ductus arte- 

 riosus are functional and they actually precede the morphological closure 

 changes. The foramen ovale is functionally closed by the apposition of Septum 

 I and Septum II. This apposition is effected by the equalization of the blood 

 pressures in the right and left atria. However, the structural closure of the 

 foramen ovale is produced by the growing together and gradual fusion of 

 the two interatrial septa. The process is variable in different human individuals, 

 and failure to attain complete structural closure of the foramen ovale occurs 

 in about 20 to 25 per cent of the cases. Functionally, this failure to close 

 may not be noticeable. On the other hand, in the heart of the kitten, failure 

 to develop a complete morphological closure by 6 to 8 weeks after birth 

 is rare. 



The morphological closure of the ductus arteriosus also is gradual. This 

 does not interfere with the relative normal functioning of the lungs for the 

 opening up of the capillary bed within the lungs together with the concomitant 

 voluminous flow of blood through the pulmonary arteries to the lungs, asso- 

 ciated with the pressure exerted at the distal end of the ductus arteriosus by 

 the blood within the descending aorta, aids the functional closure of the 

 ductus arteriosus. In some individuals, the ductus arteriosus may remain 

 open, to some degree, even in the adult. 



G. Post-hatching and Post-partum Care of the Young (fig. 380) 



Although care of the young after hatching or after birth is beyond the 

 province of this work, it should be observed that such care is characteristic 

 of birds and mammals, and is present in certain instances in fishes and am- 

 phibia (fig. 380B). In the marsupial mammals, the early post-partum care 

 of the young in the marsupial pouch of the mother is closely related to the 

 pre-hatching or pre-partum care of the young in other animal groups. In the 

 opossum, for example (fig. 380D), the utterly helpless young are firmly at- 

 tached to the nipples of the mother for about 50 days (McCrady, '38). This 

 attachment in reality constitutes a kind of "oral placenta." From this view- 

 point, the care of the developing embryo in marsupial mammals may be 

 divided into two phases, namely, a uterine phase and an early post-partum 



