UTERINE BLOOD FLOW 



1593 



4 0.5 0.6 0.7 



PERIOD OF GESTATION IN TENTHS OF TOTAL 



-100 



fig. 8. Transfer rate of sodium during gestation in six species. [From Flexner & Gellhorn (81).] 



ventral, the other on the dorsal aspect of the interior 

 of the uterus. However, only one of these receives the 

 umbilical vessels and is called, therefore, the primary 

 placenta. The secondary placenta receives its vessels 

 from continuations of a number of umbilical vessel 

 branches on the chorioallantoic surface of the primary 

 placenta that pass between the amnion and chorion 

 laeve to the secondary placenta, rather like the con- 

 nection between the cotyledons of the ungulate 

 placenta. 



The "unitary" structure of the fully developed 

 discoidal placenta, of the human, rhesus monkey, 

 and other species is the cotyledon [Wilkin (245)]. 

 This is a vascular unit which is fetal (see further 

 discussion below). By implication, the maternal 

 placental structures are fitted to the cotyledon. In a 

 sense they are. The decidual plate with its septa blocks 

 off smaller areas of the fetal portion of the placenta as 

 ridges or folds of tissue about a number of the cotyle- 

 dons. However, the ridges of the septa do not make 

 connection with the fetal tissues in a way that makes 

 discrete, unitary compartments, or chambers. The 

 maternal vascular compartments interconnect deep 

 in the placenta beneath the chorioallantoic plate of 

 the placenta, so that there is a continuum of the 

 maternal vascular area. The entire area is spoken 

 of as a maternal lake or intervillous space. To the 

 extent that there is continuity, this is true, but there 

 are innumerable attachments of fetal vessels covered 

 with chorionic tissue to the basal part of the placenta 

 in the lake on the decidual plate and on the septa 



[Wilkin (245)]. The crypts or pockets between septal 

 folds become more numerous as pregnancy advances. 



In early pregnancy, isolated lakes of maternal blood 

 in the trophoblast merge and fuse as one may imagine 

 pockets of gas in aging cheese might fuse to form 

 larger pockets. This involves the entire syncytial 

 trophoblast in the area of implantation, embedded in 

 the decidua basalis. As the placenta enlarges and 

 undergoes morphogenesis and the uterus enlarges 

 pari passu, the characteristics of the placenta change 

 until it is complete in form, after the fourth month of 

 pregnancy. Only a small proportion of the interior of 

 the uterine surface then is involved. Zonary placentas 

 completely surrounding the fetus are found in some 

 species. These and still other patterns of gross struc- 

 ture and vascular arrangements have been described 

 by Mossman (158) and by Amoroso (5). 



Because there is such a variety in the types of 

 placentas and since these undergo important struc- 

 tural changes during pregnancy, a physiologist who 

 measures uterine blood flow during pregnancy may 

 not properly speak of blood flow except as the latter 

 relates to an evolving set of morphological relation- 

 ships with respect to a given form of placenta. It is 

 to be borne in mind also that the largest part of the 

 gravid uterus is not associated with the placenta; it, 

 too, must be supplied with blood. There are, in a 

 sense, two uterine blood flows, one to the placenta in 

 its various forms, the other to the uterine tissues which 

 are undergoing enormous growth, stretching and 

 change of shape. To measure total uterine blood flow 



