38 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the larval and the perfect state, has continued the investigations of 

 earlier naturalists by a study of Musca vomitoria. When the larva 

 becomes converted into the pupa, the skin of the whole of the body, and 

 not only that of the head and thorax, undergoes degeneration of the 

 hypodermic cells ; and this is carried so far, that at one time the 

 animal has nothing but a delicate cuticle covering it ; the embryonic 

 cells which fill nearly the whole of the body of a pupa are not all 

 derived from the nuclei of the muscle-cells, some are formed by the 

 proliferation of the cells of the fat-body. It is pointed out that the 

 return of the tissues to the embryonic condition is the cause of the 

 pupa having, at a certain time, really the characters of an embryo ; 

 if we make a section across the abdomen of a pupa between the second 

 and fourth days we see that it is only composed of two layers of 

 central cells, one formed from the epithelial cells of the digestive 

 tube and the other, set peripherally, and formed by embryonic cells 

 derived from the muscular nuclei and the cells of the fat-body. The 

 imaginal disks seem at first to form a hollow sphere in which one 

 part has been pushed into the other ; the inner layer is thick, and 

 made up of pyriform cells, the outer layer is delicate, and its cells 

 flattened. Later on, the latter disappear, and the inner layer gives 

 rise to the integument of the adult. The disks for the eyes are 

 distinguished by having the cells of their inner layer regularly set 

 side by side ; they are cylindrical in form, and their inner extremity 

 is pointed ; by this they become connected with the fibrils of the 

 optic nerve. The author finds that the integument of the abdominal 

 region of the adult is formed by the conversion of the embryonic into 

 hypodermic cells, the hypoderm first appearing, for each joint, at two 

 superior and two inferior points. Further observations are promised 

 which will deal with the metamorphoses of the nervous system. 



Criticizing the statements put forward by Viallanes, Kunckel* 

 points out how the author's view, that the embryonic cells are partly 

 formed by proliferation of cells of the fat-body, has been contradicted 

 by his own observations and those of Ganin, which show this body 

 to be no more than a reserve of nourishment, in other words, a 

 post-embryonal vitellus. The buds which give rise to the integument 

 of the head and thorax have been wrongly termed " histoblasts " by 

 Viallanes, for they have not, as the term would imply, a common 

 origin and common constitution, but give rise to the nerves, tracheas, 

 and the skin itself; their structm-e has been rightly elucidated by 

 Ganin as that of small sacs filled with cells, and as having an exoderm 

 and mesoderm. The development of the hypodermic cells, as 

 already described by Ganin, is essentially the same as that described 

 by Viallanes. 



A justification by Viallanes of his statements is given in a sub- 

 sequent paper,| in which he points out that Ganin regarded the 

 hypodermis of the abdomen of the adult as being developed by trans- 

 formation, while he has proved that there is a true degeneration of 

 the cells. 



* Comptes Rendus, xciii. (1881) pp. 901-3. 

 t Loc. cit. pp. 977-8. 



