DIGESTIVE ACTIVITY OF MESENCHYME 469 



small lymphocyte, by this hypothesis, is responsible not only for 

 the resistance which is stated to have been obtained against 

 tumor in an embryonic allantois by the introduction of adult 

 spleen, but also for the resistance against heteroplastic grafting 

 which develops naturally in the organism. On the basis of the 

 fact that tumor grafts do not take at the eighteenth to the nine- 

 teenth day of incubation, and that a growing tumor recedes at 

 this time, Murphy believed that a general resistance in the 

 chick embryo appears during the last days of embryonic life. 

 He emphasized, however, in his paper the failure to find any 

 noteworthy change in the organism which could account for the 

 appearance of this resistance. He later attributed the resistance to 

 the appearance of small lymphocytes in the spleen of the embryo. 

 It would seem that this theory concerning the small lympho- 

 cyte as bringing about and maintaining an immune state in the 

 organism cannot be sustained. As presented in section three of this 

 paper, the recession of the tumor in the allantois at the eighteenth 

 to nineteenth day of incubation, or its failure to take if grafted 

 at this time, is sufficiently explained by local changes in the 

 membrane itself. At that time these changes affect any tissue, 

 even those from a chick of the same stage as the host itself. It 

 should be remembered also that the spleen, according to Ton- 

 koff, develops at the fourth day of incubation, and that the small 

 lymphocytes, according to Danchakoff, begin their differentia- 

 tion in this organ at about the fifteenth day of incubation. 

 Small lymphocytes, however, appear in the blood current from 

 about the twelfth day of incubation because of their develop- 

 ment in the thymusv-^ No directly injurious effect upon the tumor 

 cells, however, by any element of the spleen of animals younger 

 than one to two weeks has ever been observed. All this tends 

 to show that the development of the general resistance of the 

 chick against heterogeneous grafting cannot be demonstrated 

 at a period before birth, that in no case is the appearance of the 

 small lymphocyte in the organism responsible for the develop- 

 ment of such a resistance, and that up to two weeks after incu- 

 bation none of the cellular elements of the embryonic spleen is 

 capable of either inhibiting tumor growth or injuring tumor 

 cells. 



