no DISCOVERY REPORTS 



have not been arbitrarily selected, but are part of the normal development of the reproductive system. 

 I have dissected 3073 adolescent male specimens and have found that growth proceeds by the addition, 

 in constant sequence, of definite anatomical structures. The appearance of each structure in turn 

 marks a stage, but internal and external development do not necessarily keep pace with one another. 

 These stages, five in number, tabulated below in Table 3, are collectively equivalent to the stage 1 of 

 Ruud. Stages A-E are the approximate external equivalent of stages 1-5. 



The majority of adolescents of stage 1 show no differentiation of the copulatory organs : that is to 

 say, externally, development corresponds to stage A, the ist pleopod being unmodified. In some cases, 

 however, external growth proceeds more rapidly than internal, and the pleopod may carry a primitive, 

 undivided petasma, stage B. The total number of adolescent males of stage 1 which I have dissected 

 is 121 1, and of these 10 14 were at stage A externally and 197 at stage B. 



* Neglect? 

 Average length of each stage per annum in mm. for the whole year 



Individual length varies very considerably. Table 4 gives the monthly minimum and maximum 

 lengths for all stages, and shows clearly that size alone is not necessarily a reliable criterion of develop- 

 ment. In November, for example, the length of the individuals of stages 1 and A varies widely over 

 a range of 10-30 mm., whereas in May the range is much narrower, lying between 26 and 31 mm. 

 Nor is size always associated with a more advanced stage of internal development : in February, for 

 example, a specimen measuring 39 mm. has a pleopod at stage B, but in May the largest specimen, 

 measuring 31 mm., is only at stage A, while the small specimen (26 mm.) is already at stage B. 



The average monthly length of the specimens, on the other hand, shows a steady increase throughout 

 the year, starting at 13 mm. in August and reaching 28 mm. by the following May (see Table 5, which 

 also gives the average length of each stage for the whole year). 



This great variation in the size of individuals within a stage is characteristic of all adolescents, 

 whether they are at stage 1 or at stage 5, or at any of the intermediate stages. By referring to Table 4, 



