150 H. H. NEWMAN 



known may be assumed to be exactly equivalent to the maternal, 

 which is known. A study of the mothers, therefore, should reveal 

 the essential facts of inheritance, although the problems would be 

 more readily handled if we knew the characters of both parents. 

 Moreover, the present scheme of studying the inheritance from 

 the maternal side alone offers many unique advantages that could 

 not be duplicated under the conditions of experimental breeding. 

 It is possible readily to obtain a large number of sets of offspring, 

 numbers that could be secured only at enormous expense, if at 

 all, were breeding attempted. Again it is possible by the methods 

 employed in this study to obtain, by killing the mothers, a defi- 

 nite orientation of the fetuses in pairs and to preserve these re- 

 lations by means of their placental attachments. It is hardly 

 probable that one would be willing to kill the mothers used in 

 breeding experiments, for this would involve too great a sacrifice 

 of the time and labor expended in domesticating and bringing 

 about the mating of captive animals. If, moreover, mothers are 

 allowed to give birth to young, all placental attachments are lost 

 and the value of the material is correspondingly lessened. Finally, 

 it has been my experience that, whenever mothers give birth to 

 litters in captivity, they eat or otherwise mutilate some or all of 

 the new-bomi young, a perversion of maternal instinct that could 

 scarcely, be obviated since births seem to occur only at night. On 

 the whole then I am convinced that the present method of collect- 

 ing pregnant females at the proper season, removing and orienting 

 the advanced sets of quadruplets, preserving the complete armor 

 of the mothers, and carefully tagging mother and fetuses with cor- 

 responding numbers, furnishes the only practicable means of 

 obtaining in adequate abundance the necessary material for the 

 study of heredity in this species. Since only the armor of the 

 mothers is preserved the scope of the investigation is limited, but 

 there can be no question as to the many advantages offered by 

 the armor. The structures and arrangements are clearly defined 

 and readily enumerable. They reach a definitive number and 

 alignment comparatively early in fetal life and are so well defined 

 in advanced fetuses that the detailed comparison between mother 

 and young is easy and certain. 



