When animals of the same species are mated inter se it may usually be assumed that they will be fertile and that their progeny will be indicative of a random union of gametes. Dobrovolskaia-%am-adskaia and KoboxoefT ( '32) and later Koboziefl ( '35) have described t~7 0 mutations, t" and t', having ti11 apparent effect 011 segregation in the male mouse (Chcsley and Duiin, '36 ; Dunn and Gluecksohn-Schoenheimer, '39).Males heterozygous f o r either t" or tl transmit the mutation to their of'fspring with a frequency significantly greater than may be expected on an orthodox Mendelian basis.Both to and t' are at the locus of Brachy ( T ) , the factor for dominant short tail discovered by nobrovolskaia-Za~~~a~slrRia ( '27) and studied embryologically in the hcterozygous and lethal homozygous condition by Chesley ( '35). If t' is combined with T the resulting mice are tailless. When mated together T/tl animals (line 29) produce only tailless progeny through the operation of a system of balanced lethals as first described by Muller ( '18) in Drosophila. T/to (line A) mice are also tailless and true breeding. Males with normal tails (to/tl) may be obtained by crossing line A by line 29 and are com-'Submitted in partial fulfillment of the requirements for the degree of Doctor