102 CARNEGIE INSTITUTION OF WASHINGTON. 



An examination of tetraploid somatic cells in Sarcophaga has indicated that 

 in prophases of these cells all four homologous chromosomes of each sort 

 come into intimate and equivalent association, just as do the two homologues 

 in diploid cells. 



In addition to the above, the comparative survey of chromosomal conditions 

 in the Diptera has been continued, particularly by Miss M. S. Moses. 



EXPERIMENTAL MODIFICATION OF THE GERM-PLASM. 

 Genetic Behavior op X-rayed Mice. 



In the Year Book for 1921 reference was made to the experiments of Drs. 

 C. C. Little and H. J. Bagg, in exposing mice to very small doses of X-rays. 

 The dosage given was one-fifth of an erythemal dose for five successive days. 

 The radiation was given over the whole body to both male and female parents. 

 This part of the experiment was performed at the Memorial Hospital in New 

 York City. 



The further experiment involved the contrasting of the breeding behavior 

 of these mice with that of untreated controls. Dr. Little reports : 



"The first litters of young to be recorded from any of the treated animals 

 did not appear until at least six weeks after radiation. As a general thing, 

 these litters were slightly smaller than the normal litter size, although the 

 young produced were apparently normal and healthy. These animals and 

 their descendants have since been bred inter se with brother-to-sister matings. 

 More then 4,500 mice have been recorded in these experiments. Approxi- 

 mately 1,000 of these are descendants of the untreated controls. Among these 

 have appeared two grossly abnormal individuals, both in a single, unusually 

 large litter. Experiments are now being conducted to determine whether or 

 not this tendency to produce abnormals is hereditary, and if so, what the type 

 of inheritance is. Information on this point should be obtainable within the 

 next year. Among the descendants of the X-ray pairs, more than 275 grossly 

 abnormal individuals have been obtained. It has been possible to group these 

 abnormalities somewhat as follows : 



"(1) Eye lesions. — These are usually hemorrhagic, involving a dry or 

 crusted appearance of one or both eyes. This abnormality is clearly visible 

 at birth and persists throughout the lifetime of the individual, usually pro- 

 ducing blindness due to clouding atrophy, and in rare cases apparently com- 

 plete absence of the e3^e. The inheritance of this abnormality has been care- 

 fully studied, and it has been found to be Mendelian in its behavior and 

 recessive in nature. The recessives, when bred together, do occasionally give 

 individuals somatically normal in appearnace, but these are merely somatic 

 "overlaps" and breed as abnormals. Since the type of lesion appearing in 

 this abnormality usually involves hemorrhagic areas, the symbol h has been 

 given to this mutation, with H as its normal allelomorph. 



" (2) Abnormal head. — This abnormality involves a series of different 

 types. The commonest of these shows defects in, or absence of, the lower jaw. 

 Correlated with these appears reduction or absence of the mouth opening and 

 of the tongue. Occasionally, due to the reduction of the lower jaw, the tongue 

 appears large and protruded from the mouth. Naturally these mice are either 

 born dead or die soon after birth, since it is impossible for them to nurse, and 

 in some cases to breathe. At other times the eyes are also involved, but in 

 these cases there is no lesion to the outside, as in the types described under 

 heading (1). The eyes are either merely reduced or perhaps more commonly 



