ALKALINE PHOSPHATASE AND SECRETION 71 



case with representative members of all the other vertebrate 

 classes (see Moog, 1946, for a survey of the literature). The 

 only case where there is alleged to be an absence of alkaline 

 phosphatase is with the toad fish Opsanus, one specimen of which 

 was studied by Wilmer. On the other hand, a number of species 

 of aglomerular fishes were studied by Lorch and Danielli, who 

 found that alkaline phosphatase was present in the border regions 

 of the secreting cells in all cases. It seems possible that the obser- 

 vation upon Opsanus is in error; post-mortem changes occur very 

 readily in the kidneys of fish. Whereas the detailed morphology 

 of the excreting organs of invertebrates differs very markedly 

 from that of the vertebrate kidney, the invertebrate excretory 

 organs always contain regions which have either been proved 

 to be, or are suspected to be, carrying out secretory processes 

 closely analogous to those occurring in the vertebrate kidney. 

 In all cases where these organs have been studied, these "secre- 

 tory" regions are found to be rich in alkaline phosphatase at sites 

 functionally analogous to the sites in mammalian kidney which 

 are rich in alkaline phosphatase. This is the case for the ne- 

 phridia of a terrestrial triclad (Pantin and Danielli, 1950) , for the 

 green, gland of the crayfish Camburus (Kugler and Birkner, 

 1948), for the Malpighian tubules of the tick Rhodnius, and a 

 number of insect larvae (Bradfield, 1950; Yao, 1950). 



Apart from the excretory organs, numerous other sites of 

 solute secretion contain alkaline phosphatase on the free border 

 of the secretory cells which is exposed to the fluid from which the 

 solute is to be removed. This is the case with the small intes- 

 tine, the choroid plexus, the prostate gland, the placenta, etc. (for 

 a review see Bradfield, 1950) . 



Thus there is an impressive body of evidence that alkaline 

 phosphatase is associated with the selectively active borders of 

 a wide variety of secretory cells. But it is still not clear whether 

 the phosphatase is inside the plasma membrane or on the exterior 

 of the cell. These two possibilities must be considered separately. 



If the phosphatase is on the outside of the cells, it is in the 

 position in which it can carry out the dephosphorylation A of 

 the mechanism described on p. 69. But the situation is not 

 particularly satisfactory, for there has so far been no indication 

 of the existence of the other postulated enzyme systems at B 

 and C. 



