150 SPACE FACTOR AND GROWTH RATE 



2. The depth of the culture medium (Spring Water or Standard Solution) must be 

 maintained constant. If there is evaporation, this loss should be replaced 

 with glass distilled water. The water need not be changed until the tadpoles 

 hatch, unless it becomes turbid. If the aquarium is covered with a glass 

 plate and is placed in uniform light and heat, it will require no care until 

 after the beginning of feeding. (Remove all dead eggs and embryos, record- 

 ing the fact. ) 



3. After the absorption of the external gills, begin to feed the tadpoles on the 

 standard diet of washed and softened spinach. The green leaves should be 

 washed in running water, and softened by par-boiling. Tadpoles must be 

 cleaned and fed daily thereafter. On occasion it may be necessary to re- 

 move all tadpoles to marked crystallizing dishes for a few minutes while 

 giving the aquarium a thorough cleaning, to avoid bacterial contamination. 



OBSERVATIONS AND EXPERIMENTAL DATA: 



In the beginning the record will consist merely of staging the embryos under the various 

 space conditions. After the tadpoles hatch, it will become necessary to make three 

 records from each of the groups of tadpoles at each reading. It is suggested that the 

 readings be taken at weekly intervals. The three records will be (a) size of the largest 

 tadpole (b) size of the smallest tadpole (c) average of five sizes. Such size readings can 

 be made quickly in Petri dishes over scaled graph paper, and are total length. After 

 about 2i months (at ordinary laboratory temperatures and adequate feeding) the forelimb 

 emergence will be detected in some specimens. This may be taken as the final step in 

 the experiment, i. e. , when 50% of the tadpoles reach forelimb emergence. Record the 

 data in the following manner: (See record on following page. ) 



DISCUSSION: 



During the early cleavage stages there may be no detectable difference in the rate of de- 

 velopment under the various conditions of reduced crowding. As soon as there is freedom 

 of movement the tadpoles must seek out their food and, in doing so, encounter each other 

 and they are thereby stimulated to further activity, and we begin to find differences in 

 growth rate. Even in the same clutch of eggs there may be genetic differences which 

 might explain differences in growth rate, hence it is important to compare the averages 

 and the composite averages from the compartments of the same number. The groups are 

 so arranged to minimize any differences relative to the accumulation of metabolites and 

 faecal waste. The single variable in this experiment is supposed to be the available 

 space per tadpole, all other factors of oxygen, food, light, temperature, etc. being equal 

 for all specimens. The surface area is identical for all compartments, and presumably 

 the dissolved oxygen is the same, particularly if the activity of the tadpoles causes suf- 

 ficient agitation of the medium to circulate all dissolved gases to an homogeneous condi- 

 tion. Lynn and Edelman (1936) have carried this experiment to metamorphosis and find 

 that crowding not only affects the rate of development but the success of achieving meta- 

 morphosis. The optimum conditions for development, at least in the pre-feeding stages, 

 is a ratio of 1 tadpole per 2 cc. of medium in a total of 50 cc. per finger bowl. After 

 feeding begins, this ratio would be considered as definitely crowding the tadpoles and 

 development would be consequently retarded. 



