922 



SPERM, OVA, AND PREGNANCY 



its brief life history (Padykula, 1958a, b) . 

 A major architectural reorganization occurs 

 with the loss of the parietal wall shortly af- 

 ter mid-gestation. A short period of lipid 

 storage for from 10 to 15 days is succeeded 

 by a phase of glycogen storage from 15 to 

 20 days. After the loss of the parietal wall, 

 there is a sharp rise in certain enzymatic 

 activities (alkaline phosphatase, adenosine 

 triphosphatase, acid phosphatase, succinic 

 dehydrogenase) . This burst of vitelline ac- 

 tivity in this last third of gestation suggests 

 greater functional activity after direct ex- 

 posure of the visceral endoderm to the 

 uterine contents. As in the case of the cho- 

 rio-allantoic placentas, the reduction in the 

 number of layers of the yolk sac probably 

 increases the rate of absorption by the vas- 

 cularized splanchnopleure. Shortly before 

 term there is a sharp decline in glycogen 

 content and certain enzymatic activities in 

 the visceral endoderm. As these particular 

 histochemical properties decline in the vis- 

 ceral yolk sac, they appear in the fetal liver 

 with good temporal correlation. If the yolk 

 sac functions in part as a fetal liver, then 

 the terminal decrease in enzymatic activity 

 should not be interpreted as placental aging, 

 but rather as a redistribution of the func- 

 tional activities of the placental-fetal com- 

 plex (Padykula, 1958). Further morpho- 

 logic aspects of aging in the placenta were 

 discussed by Wislocki (1956). 



With the ascendency of the chorio-allan- 

 toic placenta in eutherian mammals and its 

 postulated progression in the sense of Gros- 

 ser's series from a simple epitheliochorial 

 placenta to the physiologically more "effi- 

 cient" hemochorial type, one might have 

 expected that the mammalia would have 

 abandoned yolk sac placentation altogether. 

 But that is not the case, for paradoxically 

 the greatest placental development of the 

 yolk sac, involving inversion, is associated 

 with hemochorial placentas (rodents, lago- 

 morphs, bats, some insectivores), whereas 

 the least developed yolk sacs occur in ani- 

 mals possessing epitheliochorial placentas 

 which are the most primitive, according to 

 Grosser's scheme. The general adoption of 

 Grosser's concept of the chorio-allantoic 

 placenta has resulted in the almost complete 

 exclusion of the yolk sac. For example. 



Needham (1931, Table 227) attributes the 

 entire transfer of substances in mammals to 

 the chorio-allantoic placenta, without ref- 

 erence to other avenues of exchange. In view 

 of the observations and experiments de- 

 scribed here, it is evident that Grosser's 

 doctrine will have to be re-evaluated and 

 modified to include yolk sac placentation. 



VI. Histochemistry with Reference to 

 Comparative Placentation 



It is beyond the scope of this chapter to 

 describe in detail the structure and cytology 

 of the placentas of various mammals. How- 

 ever, certain histochemical observations on 

 lipids will be presented because they afford 

 some clues to the probable sites of localiza- 

 tion of placental steroid compounds. In ad- 

 dition, the localization of glycogen, other 

 complex carbohydrates, and phosphatases 

 in several types of placentas will be sum- 

 marized. This histochemical information 

 will serve as a basis for subsequent com- 

 parisons of placental structures and func- 

 tions. 



However, before proceeding to these mat- 

 ters, attention should be drawn, for readers 

 who may wish to familiarize themselves 

 with comparative placentation, to the com- 

 pendia of this subject by Grosser (1925b), 

 Mossman (1937), and Amoroso (1952). Re- 

 cent papers on placental histochemistry of 

 various animals will be listed here. These 

 include investigations of the chemical mor- 

 phology of the placentas of the pig (Wis- 

 locki and Dempsey, 1946b), sheep and cow 

 (Wimsatt, 1950, 1951), shrews (Wislocki 

 and Wimsatt, 1947), cat (Wislocki and 

 Dempsey, 1946a), and bat (Wimsatt, 1948, 

 1949). In rodents histochemical observa- 

 tions are more numerous, including earlier 

 investigations of fat, glycogen, and iron in 

 placentas of the rabbit (Chipman, 1902) 

 and rat (Goldmann, 1912). More recent 

 studies describe glycogen in the rat's pla- 

 centa (Szendi, 1933; Krehbiel, 1937; Bridg- 

 man, 1948; Bulmer and Dickson, I960), al- 

 kaline phosphatase m the guinea pig's 

 placenta (Hard, 1946; Nataf, 1953), and in 

 the pregnant uterus of the rat (Pritchard, 

 1947), and multiple histochemical reactions 

 in placentas of rats, mice, guinea pigs, and 

 rabbits (Wislocki, Deane and Dempsey, 



