SPERM, OVA, AND PREGNANCY 



vessels by cytolytic substances emanating 

 from the trophoblast. Further evidence of 

 a cytolytic factor was demonstrated by the 

 observation that ova placed in the eye were 

 capable of developing in close proximity, 

 but only until the most precocious one 

 among them began to implant. Thereafter, 

 the others quickly degenerated. In connec- 

 tion with the question of cytolytic sub- 

 stances produced by the placenta, it should 

 be recalled that bits of human chorionic villi 

 obtained from placentas of the first months 

 of pregnancy and grown on plasma clots 

 liquefied the medium (Grafenberg, 1909a, 

 b; Friedheim, 1929; Caffier, 19291. 



IV. The Structure of the Placental Barrier 



Two concepts have dominated the subject 

 of placental physiology. The first of these 

 states that, as gestation advances and the 

 placenta ages in a given species of mammal, 

 the placental barrier becomes progressively 

 more permeable to physiologic exchange be- 

 tween mother and fetus. The second concept 

 proposes that the placentas of mammals can 

 be arranged in an ascending order or phylo- 

 genetic series with reference to the relative 

 facility and rapidity with which metabolites 

 traverse them. 



The placental barrier can be defined for 

 Mammalia as "an apposition or fusion of 

 the fetal membranes to the uterine mucosa 

 for physiologic exchange" (Mossman, 1937). 

 The membranes involved are the chorion, 

 allantois, and yolk sac, w^hich in various re- 

 lations to one another and to the uterine tis- 

 sues give rise to a variety of placental struc- 

 tures, the nature of which will be discussed 

 below. 



A. grosser's classification 



The relative permeability of the placenta 

 in eutherian mammals, with reference to 

 both ontogeny and phylogeny, has been 

 generally related to four morphologic pla- 

 cental types defined by Grosser (1909). 

 According to this doctrine, the chorio- 

 allantoic placentas of mammals can be 

 arranged in an ascending order from the 

 most primitive to the most advanced on the 

 basis of the successive disappearance of 3 

 out of 6 layers which intervene between the 

 maternal and fetal blood streams. The dis- 



appearance of the maternal layers is at- 

 tributed to the invasive and aggressive prop- 

 erties of the trophoblast of the chorion. In 

 the most primitive placentas the layers com- 

 prise (1) the uterine vascular endothelium, 

 (2) the uterine stroma, (3) the uterine epi- 

 thelium, (4) the fetal trophoblast, (5) the 

 fetal stroma, and (6) the fetal capillary 

 endothelium. When these layers are all pres- 

 ent, they form the so-called epitheliochorial 

 type of placenta encountered in some ungu- 

 lates (e.g., pig, mare), Cetacea, and lemurs. 

 Next, the uterine epithelium disappears 

 through the invasive activity of the tropho- 

 blast, with the formation of the syndesmo- 

 chorial type of placenta of other ungulates 

 {e.g., cow, sheep). Then with the loss of the 

 maternal connective tissue, the endothelio- 

 choricd type of placenta of carnivores (dog, 

 cat), sloths (Bradypodidae), Tupaiidae, 

 some Insectivora (shrew% mole), and some 

 bats (Chiroptera) arises. Finally, with the 

 ultimate loss of the maternal endothelium, 

 the hemochorial type of placenta of ro- 

 dents, Tarsiidae, monkeys, anthroi:)oid apes, 

 and man is formed. This series has been 

 widely accepted as having phylogenetic sig- 

 nificance, in that it is sujiposed to begin 

 with a primitive, six-layered placental bar- 

 rier which is the least permeable, and to 

 l^rogress, by a successive reduction of the 

 three maternal layers and a gradual dimi- 

 nution in width of the remaining fetal lay- 

 ers, to the most advanced evolutionary type 

 in which transmission is most rapid and 

 complete. 



As an extension to Grosser's classifica- 

 tion, Mossman (1926, 1937) sought to dem- 

 onstrate that in lagomorphs and higher ro- 

 dents, especially rabbit, rat, and guinea pig 

 toward the end of gestation, the tropho- 

 blastic syncytium is normally, and very 

 generally, lost, so that the placental mem- 

 brane consists merely of fetal capillaries 

 composed of endothelium. For the rabbit's 

 placenta, he stated that after the 22nd day 

 the syncytium disappears so that the pla- 

 cental barrier becomes reduced to a layer 

 of "endothelium and a very thin Plasmo- 

 dium, the latter entirely absent in many 

 places." This he designated as a hemoendo- 

 thelial type of placenta, and it has been 

 accepted as the most advanced stage of the 



