108 George B. Wislocki 



The several enzymatic reactions were lowest at 13 days in 

 all parts of the rat's placenta. In the visceral endoderm of the 

 yolk sac, alkaline glycerophosphatase reached a maximum at 

 17 days and declined to almost none by 21 days. Adenosine 

 triphosphatase rose to a maximum at 19 days but then de- 

 clined somewhat by the end of gestation. Acid phosphatase 

 activity rose steadily until the end of pregnancy. Esterase 

 activity reached a peak at 15 days and diminished slowly 

 thereafter. Succinic dehydrogenase activity was maximal 

 on the 16th day and then declined rapidly. Assays of homo- 

 genates of the yolk sac confirmed the histological findings for 

 adenosine triphosphatase, alkaline glycerophosphatase and 

 succinic dehydrogenase. In the chorioallantoic placenta, 

 adenosine triphosphatase activity increased markedly in the 

 spongy zone between the 17th and 19th days, as did also acid 

 phosphatase between the 15th and 19th days. 



Various cytological and cytochemical changes have been 

 reported in the placenta of the rat (Wislocki and Padykula, 

 1953). For example, glycogen begins to accumulate in the 

 visceral endoderm of the yolk sac on the 15th day, reaches a 

 peak on the 18th day, and declines thereafter. A mucopoly- 

 saccharide, demonstrable by the periodic acid-Schiff reagents, 

 increases from the 10th to 16th days and then declines (18th 

 day), but increases once more by the 21st day. In the tropho- 

 blast of the placental labyrinth, where glycogen is never 

 abundant, it increases up to the 18th day and subsequently 

 diminishes. In the spongy zone and the subplacental myo- 

 metrium, glycogen is extremely abundant up to the 18th day 

 but then declines rapidly. 



The decrease of some substances in the placenta may be 

 related to the assumption of functions by various foetal 

 organs, especially the liver. From study of the rabbit's 

 placenta, Claude Bernard (1895) advanced the idea that the 

 placenta serves as a deputy for the synthesis and storage of 

 glycogen until the foetal liver begins to function. In confirma- 

 tion of this, glycogen first appears in the liver of the rat on the 

 19th day of gestation when it has begun to disappear from the 



