916 



SPERM, OVA, AND PREGNANCY 



SOW, Amoroso (1952) has described the 

 trophoblast at one period as sending actual 

 processes between and past the maternal 

 epithelium into the region of the underlying 

 maternal capillaries, establishing an endo- 

 theliochorial relationship. Despite the pres- 

 ence of six theoretic layers in the sow, pro- 

 visions exist which tend to by-pass or 

 materially reduce the width of several of 

 them, thus diminishing the distance between 

 the two blood streams. Similarly, in the cat, 

 in the last half of pregnancy, Amoroso (1952) 

 observed "that the foetal capillaries come to 

 lie so near the surface of the lamellae that 

 only the thinnest laminae of syncytial 

 trophoblast separate them from the mater- 

 nal tissues." 



Measurements of the width of the mater- 

 nal epithelium in the sow, made by Gell- 

 horn, Flexner and Pohl (1941), show that 

 its height changes from 18 fj. at midgesta- 

 tion to 10 fi just before term. Nevertheless, 

 in many of the thinner places between the 

 capillaries by midgestation (Fig. 15.23 of 

 the present study), the width of the inter- 

 vening cytoplasm of the combined chorion 

 and uterine epithelium is reduced to no 

 more than 6 or 8 fj.. In the sheep at 100 days 

 of gestation Barcroft (1947) reported that 

 none of the fetal and maternal capillaries 

 is closer than 20 ix and none is separated by 

 more than 120 /j.. This offers no clue, how- 

 ever, as to what the average distance may 

 be. It is apparent, nevertheless, from some 

 excellent figures of the sheep's placenta sub- 

 mitted by Wimsatt (1950, Figs. 54 and 56) , 

 that the numerous maternal capillaries arc 

 about 10 to 20 /x from the fetal capillaries 

 at 100 days and less at 133 days. In the cat 

 at term, the distance between the two blood 

 streams is narrowed in many places to 6 or 

 8 fji. In the human at term, the thinnest 

 places vary between 3 and 6 /x in width. 

 More extensive and careful measurements 

 of the distances between the blood streams 

 should be obtained in various animals at 

 different stages of gestation, in order to pro- 

 vide a better basis than now exists for com- 

 parisons. In making such measurements the 

 degree of shrinkage and separation of the 

 layers in preparing the tissues should be 

 carefully evaluated. 



A more important consideration than the 



actual diminution in width of the layers in 

 the thinnest regions might be the apparent 

 much larger extent of thin areas in hemo- 

 chorial placentas than in other placental 

 types. Thus, for example, although the thin- 

 nest places in the sow's placenta do not 

 seem to differ greatly from the human in 

 respect to their actual widths, the relative 

 extent of the thin areas is very much greater 

 in the latter than the former. In this re- 

 spect, hemochorial placentas differ greatly 

 from epitheliochorial ones. This considera- 

 tion, although possibly inherent in Gros- 

 ser 's doctrine, has never been clearly 

 brought out and documented, but instead 

 has been subordinated to the prevailing con- 

 cepts of the phylogcnetic reduction in num- 

 ber and widths of the layers. 



A further point of interest concerns the 

 placentas of rodents. The physiologic ad- 

 vantages obtained presumably by the re- 

 duction in width of the trophoblastic mem- 

 brane and the increased extent of the thin 

 areas would seem to be offset by the func- 

 tional disadvantage of the laminated ar- 

 rangement of the trophoblastic cells as re- 

 vealed by electron microscopy. Here, where 

 a syncytium with only inner and outer sur- 

 faces was believed to exist, the trophoblast 

 is laminated, so that 4- or 6-cell surfaces 

 extend across the placental barrier. Thus, 

 with respect to cell surfaces and cell layers 

 forming the placental barrier, the hemo- 

 chorial placentas of rodents seem to be 

 quite as complex as epitheliochorial and 

 syndesmochorial placentas. The most im- 

 portant difference between them would 

 seem to lie in the relatively greater extent 

 of the thin regions, rather than in any ex- 

 treme reduction of the number of cell layers 

 in the rodent's hemochorial placenta. 



Some degree of cytologic and histochemi- 

 cal simplification is apparent in successive 

 stages of gestation in any given species, but 

 striking cytologic differences between the 

 placental membranes of Grosser's phylo- 

 genetic series at equivalent stages of gesta- 

 tion are not very evident. Even the thinnest 

 regions of the different types of chorio-al- 

 lantoic placentas possess a far greater cyto- 

 chemical complexity than the glomerular 

 and pulmonary membranes. Several writers 

 have postulated that toward the end of 



