112 TIIK AMEBIC AX XATURALIST [VoL.LV 



placental transmission, though we are going to practice 

 immunization generation after generation for a number 

 of generations to determine if a truly hereditary im- 

 munity will be established. However, facts have come to 

 light which show that there is more concerned in the 

 operation than a mere transfer of antibodies from 

 mother to fetus. For instance, the blood of young shortly 

 after birth may show a higher titer than that of the 

 mother. Again, after two or three months of develop-, 

 ment the young of certain of the sensitized mothers have 

 shown a rather sudden rise in titer, much above that of 

 the mothers. In such cases it would seem that some 

 mechanism in the young rabbit itself is constructing 

 antibodies w^hich supplement those passively derived 

 from the mother. Possibly in the process of develop- 

 ment some organ important in such reactions just came 

 into functioning. If this is true further experiments 

 may throw some light on the perplexing question of 

 the source or sources of the antibodies in an animal. 

 After a few weeks, in such cases, the titer drops back 

 again. In still another set of experiments we found 

 that young from a sensitized mother, when nursed by 

 a normal untreated mother, retained a fairly high titer 

 for several months and even showed the rise of titer 

 mentioned. On the other hand, young of an untreated 

 mother when nursed by a sensitized mother acquired 

 a fairly high titer from the milk of the foster mother 

 but lost it rapidly after weaning time. Thus there are 

 evidently constitutional factors operative in the young 

 which have acquired their immunity through the placenta 

 which are absent in the young whose antibodies were 

 conveyed through food. 



That changes in the blood serum may be caused by 

 changed conditions in the tissues is further attested by 

 many facts. For example, in pregnancy, the newly form- 

 ing placenta may set free cells or cell-products which, 

 sometimes at least, cause changes in the blood-serum of 

 the mother, though the exact nature of these changes is 



