24 STORRS AGRICULTURAL EXPERIMENT STATION. 



distinguishable tj'pe is carefull}^ counted bj^ the use of a lens, 

 provided the total number of colonies on the plate is not too 

 great. Where the number of colonies on the plate is only 

 three to four hundred it is quite easy to calculate the number 

 of each type accurately. It is usually found that when the 

 numbers of all of the different .species distinguished are added 

 together they do not produce a total equal to the whole num- 

 ber of bacteria that may be found by counting all the colonies. 

 There have been left, in most experiments, a certain number 

 of colonies ^vhich have not been recognized as belonging to 

 any of the differentiated types; these have been included in 

 the tables in the column headed "undetermined." The bac- 

 teria included in this column are probably in most cases the 

 same species as those that are classified under the other col- 

 umns, but colonies which have not developed sufficiently for a 

 sharp differentiation. In plates which it has been necessary 

 to study when only two days old the number present in the 

 column of undetermined is greater than it is in the older plates, 

 for this reason. 



Where study is made of milk that is somewhat older and 

 the number of bacteria consequently considerably larger, the 

 method of calculating results is slightly different, due to the 

 vast preponderance of one species, B. acidi ladici. The dilu- 

 tions which have been used in old milk and cream have been 

 very much greater than those in the fresh milk (sometimes 

 250,000); nevertheless the number of colonies on each plate 

 is always greater. In the study of these plates we have first 

 calculated the total number of colonies that can be counted. 

 Then each plate is carefully studied with a lens and a micro- 

 scope, and every colony on a plate which is different from the 

 B. acidi ladici colony is counted and tabulated. A careful 

 study thus enables us to determine, with close accuracy, every 

 colony that does not belong to the typical No. 206. It has 

 then been assumed that the rest of the colonies belong to this 

 species, and their number determined by calculation. The 

 number thus determined is quite probably somewhat too high 

 in every case, since it is almost certain that some of the col- 

 onies of the miscellaneous bacteria escape observation and 

 would be included in the column of No. 206. We are con- 

 vinced, however, that this error is not very great, inasmuch as 



