GRADIENT-FIELDS IN POS T- EMBRYO N IC LIFE 371 



anteriorly and posteriorly to a region of high growth-intensity can 

 only be explained by postulating some polarised agency connected 

 with growth-regulation, which extends through the body as a 

 whole (fig. 179). 



3rd. 



1st. 



2nd. 



3rd. 



4th. 



5th. 



Mxpd Pereiopods 



Fig. 179 

 Polarised effect of the presence of a region of high growth-rate upon the general 

 growth- gradient, in the prawn Palaemon carcinus. Abscissae, growth-rate (per- 

 centage increase for a hundred per cent, increase in carapace length) of linear 

 dimensions of the organs represented along the ordinates : third maxilliped, and 

 first to fifth pereiopods. The large claw is here the second pereiopod (not the first 

 as in crabs and lobsters), and is much more enlarged in males (solid line) than in 

 females (dotted line). Correlated with this, in males the appendages posterior to 

 the large claw show an increased growth-rate (the increase diminishingposteriorly) , 

 those anterior to it a decreased growth-rate. (From Huxley, Problefns of Relative 

 Grozuth, 1932, after Tazelaar.) 



24-2 



