Vol. XXIV. May, i 9 ij. No. 6. 



BIOLOGICAL BULLETIN 



SOME ANOMALIES IN THE GESTATION OF THE 

 ALBINO RAT (MUS NORVEGICUS ALBINUS). 



HELEN DEAN KING, 

 THE WISTAR INSTITUTE OF ANATOMY AND BIOLOGY. 



In the course of a series of inbreeding experiments with the 

 albino rat which have been in progress for several years, records 

 have been made of the births of over 700 litters containing 

 some 5,000 individuals. These records show a number of striking 

 irregularities in gestation which are deemed worthy of note, since 

 at the present time the albino rat is extensively used as a labora- 

 tory mammal in many of the large institutions of the country. 



As the data given in this paper form part of the permanent 

 records on file at The Wistar Institute, it has seemed advisable 

 to refer to individual rats by the numbers that will identify 

 them on the record cards, instead of employing some conventional 

 lettering. The scheme for designating the rats, which is outlined 

 below, has been found very satisfactory for keeping track of a 

 large number of individuals, since it tells at once the pedigree 

 of any particular animal. In the scheme of marking used the 

 serial letter, A or B, indicates that the individual was a descend- 

 ant of one or of the other of the two females, A and B, with which 

 the experiments were started in the spring of 1909. The serial 

 letter is preceded in all cases by a number which shows the genera- 

 tion to which the rat belonged. An index number, following the 

 serial letter, indicates in which of its mother's litters the animal 

 was born: if no index number is present the rat was a member 

 of its mother's first litter. The subscript following the serial 

 letter is the number that serves to distinguish each particular rat 

 from the other individuals belonging to the same generation and 

 litter group. An illustration of this method of designating the 



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