^°^ DISCOVERY REPORTS 



our studies of the problems of growth, it has gradually become manifest to us that, with our present 

 knowledge of the numerous factors determining growth, the problem of finding a law that will explain 

 the whole Cham of causes upon which growth depends is for the time being insoluble." 



ACKNOWLEDGEMENTS 

 I am fortunate in having been able to discuss various problems arising during the course of this 

 work with Dr N. A. Mackintosh and with Dr F. C. Fraser, and I am very grateful for their criticism 

 and advice. My colleagues, Dr T. J. Hart and Miss D. M. E. Wilson, have helped me in many ways 

 the former by his work on the phytoplankton of the Antarctic zone, and the latter by her constant 

 interest and the practical way in which she has helped me to reduce the large body of evidence into 

 manageable shape. 



DEVELOPMENT 



LARVAL KRILL 

 During the first year of growth, Enphausia mperba passes from the egg through the successive larval 

 stages of nauphus, metanauplius, calyptopis and furcilia, umil it enters upon its second year of post- 

 larval or adolescent life. 



Its larval history has been dealt with in detail by Fraser (1936) in his paper on the "Development 

 and distribution of the young stages of krill {Euphamia mperba) ". A summary of his work and a com- 

 parison with the observations of Taube and Lebour on euphausians of the northern hemisphere must 

 be given here, in order to present as complete a record of early growth as possible. 



Fraser obtained, by analysis of plankton samples, records of eggs and their occurrence extending 

 from the first part of November to the latter part of March, a period of four and half months Just 

 before laying, the eggs of E. superba are so tightly packed that, on the outer surface of the ovary they 

 are approximately pentagonal or hexagonal in shape, while on the inner side they are roughly conical 

 I have measured sixty eggs from two gravid females, and I find that their average diameter is 0-55 mm 

 although their greatest diameter may be as much as o-68 mm. or even 072 mm. (Ruud, 1932), but that! 

 after laying, the eggs assume a spherical shape with a consequent adjustment in size, those' found in 

 the plankton and examined fresh measuring o-6o mm. 



Fraser states that "eggs occurred in the plankton showing all stages of development, culminating 

 m the clearly distinguishable form of the ist nauplius". Only two free-swimming specimens of the 

 ist nauphus were obtained, one measuring 0-63 mm. in length and the other o-66 mm. These were 

 caught during the second half of December, together with three 2nd nauplii, measuring 0-65 o-68 

 and 070 mm. respectively. "The rarity of ist and 2nd nauplii and the smallness of numbers where 

 records exist may mdicate that these stages are passed through very rapidly in this species, as in other 

 euphausiids where the development is known". 



Taube (1915) and Lebour (1926) found that in northern waters the euphausian egg can develop into 

 the metanauplius within a few days. Observations on Nyctiphanes norvegicus indicate that the free- 

 swimming nauplius is hatched from the egg three days after laying, and that by the fifth day the limbs 

 have taken on the metanauplius form, but that the mandible and lower lip characteristic of the fully 

 developed stage do not appear until about the fourteenth day. 



Metanauplii occurred in the Discovery material in fair numbers from February onwards very big 

 catches being obtained at two stations in March. The average length of the larvae at this stage is 

 approximately 0-95 mm. 



The measurements of these early developmental forms show that the larvae do not grow very 



