310 THE DEVELOPMEM' OF THE NYMPH. 



the relations of fats to proteids are far from being understood 

 in man and the higher animals. 



Histolysis of the Sericteria. — Much has been written on the 

 degeneration of the cells of the lingual (salivary) glands of the 

 larva, which are removed by histolysis on the third day of the 

 pupa stage. Kowalevski describes the process as precisely 

 similar to that observed in the muscles, and says that no organ 

 exhibits it more beautifully. I have not been so fortunate in 

 the investigation of this subject as Kowalevski, and, although 

 I have no doubt that they are removed by phagocytes, I have 

 not succeeded in obtaining such good preparations from these 

 cells as from the muscles and fat bodies. It is well worthy of 

 remark that the histolysis of these glands takes place at a later 

 period than that of the other internal organs. This fact 

 appears to me to indicate that the Muscidie are descended 

 from insects in which the sericteria were concerned in the 

 formation of a cocoon. 



Causes of the Immunity of the Imaginal Tissues. — The question 

 has often been asked why certain tissues, those proper to the 

 larva, are attacked by phagocytes, whilst those which are 

 destined to develop the imago possess an immunity to their 

 action, although surrounded by them. It cannot be said that 

 this question has been satisfactorily answered. Barfurth [144J 

 came to the conclusion that those tissues only are attacked in 

 which degenerative changes have already commenced. 



Metschnikoff [143] observed that the leucocj'tes of a rabbit 

 do not enclose the virulent bacilli of splenic fever, but that 

 when the virus of these bacilli is modified by cultivation, they 

 are enclosed by the same leucocytes in great numbers. He 

 concluded, therefore, that the poison of the virulent bacilli 

 protects them from the action of phagocytes, and, further, as 

 certain poisons {leucomaittcs) are formed in living and func- 

 tionally active cells, he postulates that the tissues destined to 

 form the imago are endowed with such substances, and are 

 so protected from the action of phagocytes. The theory is 

 certainly ingenious, but would require additional evidence in 

 its favour before it can be received. 



