THE DEVELOPMENT AND LIFE-HISTORY OF KRILL iii 



the whole range of size can be seen, while Table 6 gives the number of males and females in each stage 

 per month and their degree of external development. 



Of stage 2, 795 specimens were examined. The stage of external development of the majority of 

 these (694) was stage B, though in 60 specimens the copulatory organs were as yet undeveloped 

 (stage A). Among the largest specimens, more advanced external development was found, 40 specimens 

 being at stage C and one even at stage D. This last specimen was not, however, the largest recorded; 

 it measured 43 mm., whereas the greatest length found was 46 mm., this specimen occurring in March 

 and being at stage C externally. The average length of stage 2 was 26 mm. at the beginning of the 

 southern spring and a steady increase was maintained throughout the year, rising to a maximum of 

 33 mm. in March, with a slight drop to 32 mm. in June. 



No specimens at stage 3 were found showing the primitive condition, stage A, of external develop- 

 ment. The majority (268) were at stage C, iii were at stage B, 150 at stage D, 36 at stage E and 5 at 

 stage F (the first external stage usually characteristic of young adults), making a total of 570 in all. 

 External development is beginning to run ahead of internal development, with the result that super- 

 ficially some of the specimens appear to be approaching maturity. The maximum lengths in some 

 months are similarly deceptive, in December to February the largest specimens being over 50 mm. The 

 average monthly lengths, however, indicate steady growth to a size intermediate between young 

 adolescents and adults ; they increase from 32 mm. in August to 44 mm. in March, with a fall to 37 mm. 

 in June. 



Among the 311 males at stage 4, much the same conditions obtain. The largest specimens were 

 well advanced in external development, being mostly at stages E and F, although the biggest one of all, 

 measuring 59 mm., was only at stage D. The majority of the specimens (150) were also at stage D, while 

 two specimens were at stage B, 47 at stage C, 85 at stage E and 27 at stage F. The average monthly 

 length of stage 4 increases from 35 mm. in August to 49 mm. in February, to fall again through 47 and 

 46 mm. to 41 mm. in May and June. 



Stage 5 is the last adolescent stage. After passing through it, the male specimens of E. superba can 

 be regarded as being fully mature. In all, 186 specimens were examined, 19 being at stage D, 97 at 

 stage E, 68 at stage F and 2 at stage G, externally these last two being fully developed, although not 

 the largest specimens measured. The average length varies from 40 mm. in August through 37 mm. 

 in October to a maximum of 53 mm. in February and April. 



ADOLESCENT FEMALES 



No special sequence of structural additions marks the growth of the female reproductive system. It 

 has been pointed out already that Ruud used the diameter of the eggs as the criterion of development 

 in the female. There is, however, a period of growth before the eggs themselves can be measured under 

 an ordinary binocular dissecting microscope, during which time the ovary is clearly getting larger. 

 I have found that during this period, it is possible to use the size of the ovary as an indication of 

 maturity. Three stages are passed through before the eggs become of measurable size: these three 

 stages mark the period of adolescence. The thelycum, the thoracic pouch into which the spermato- 

 phores are inserted, also passes through three stages before the adult condition is reached. These 

 stages of growth are tabulated below. They are based on the examination of 2933 adolescent females. 

 The primitive, unlobed, saddle-shaped ovary can be distinguished in specimens as small as 10 mm., 

 in which there is as yet no sign of the thelycum. It can also occur in specimens measuring 45 and 

 46 mm., in which the thelycum is at stage C. The average monthly length of the adolescents of stage 1 

 ranged from 13 mm. in August through 33 mm. in the following March to 30 mm. in June. The 

 thelycum was undeveloped in the majority of the specimens. Out of 1796 examined, 1150 were at 



