INTENSITY IN DIFFERENT PLANES OF SPACE 95 



smaller absolute size than is necessary in most crabs. (Fig. 

 52.) (See Huxley, 1931B.) 



During growth, the young female type of abdomen, which 

 with its flat or concave margins resembles the male's, is con- 

 verted into an almost circular structure, with the fourth and 

 fifth segments the broadest. Casual inspection would indicate 

 that one of these segments must contain the growth-centre ; 

 but casual inspection is wrong — the terminal segment (telson) 

 is so small in the young female that for it to achieve its only 

 moderate definitive size it must, and does, contain the growth- 

 centre (Figs. 52, 53). 



In the heterogenic growth of the face of mammals, which 

 we have noted in Chapter I, it would appear from the work 

 of Todd (1926) that in the palatal region the centre of maximum 

 growth lies in the palatal processes of the maxillae ; there 

 is a moderate amount of growth in the premaxillae and very 

 little in the palatals. It is impossible to arrive at quantitative 

 expression, but the facts are consonant with the idea of a 

 double gradient culminating within the maxillae. 



§ 6. Graded Growth-intensity in the Different Planes 



of Space 



So far, we have been considering growth-gradients referring 

 either to weight, or to linear measurements in one axis. 



In Pinnotheres, measurements were taken both for the 

 length and breadth of the abdominal segments (Fig. 53) ; 

 when we consider the two sets of measurements in relation 

 to each other, we find that the ratio between growth-coefficient 

 for breadth and that for length is as follows for the distal 

 half of the female abdomen : 



Thus the difference between male-type and mature female- 

 type abdomen is brought about (1) by greater growth in the 

 female, both in length and in breadth ; (2) by the breadth- 

 growth being throughout higher than the length-growth ; 

 (3) by the excess of breadth-growth over length-growth being 



