184 



T. G. Taylor, A. Williams, J. Kirkley 



was almost completed (C samples), and the sharp fall during the subsequent 4 hour 

 period confirmed the results obtained previously (Fig. 1). The expected fall in the 

 level of plasma alkaline phosphatase during shell calcification from the peak observed 

 4 hours after the start of shell formation (B samples) was also confirmed. 



Table 1. Activities of plasma acid phosphatase for individual hens at 3 stages of the laying 

 cycle expressed as iiM p-nitro-phenol phosphate hydrolysed per minute per litre plasma 



Mean 



14.2 



27.4 



16.4 



Table 2. Activities of plasma alkaline phosphatase for individual hens at 3 stages of the 

 laying cycle expressed as iiM p-nitrophenyl phosphate hydrolysed per minute per litre plasma 



Hours after start of 

 shell calcification 



Stages of shell calcification 



Mean 



Discussion 



The time of maximal alkaline phosphatase activity in the plasma (Fig. 1) corre- 

 sponded very closely to the time in the egg cycle at which Stringer (1962) observed 

 peak osteoblastic activity in the medullary bone (3.5 hours after the start of shell 

 calcification) and the subsequent decline in enzyme activity paralleled the reduction 

 in osteoblast population. A similar parallelism occurred between the increasing level 

 of plasma acid phosphatase which was observed as shell formation advanced and the 



