62 Miscellaneous. 



thorax of the adult disappear, but the skin of the whole body is 

 destroyed, in consequence of a degeneration of the hypodermic 

 cells, to such an extent that at a certain moment the animal is only 

 limited by a thin cuticle, beneath which is a thick layer of embryo- 

 nic cells, originating, as described by me in a previous communication*, 

 from the muscular nuclei which have proliferated, and before the 

 invasion of which the contractile substance of the muscular fibres 

 has disappeared. 



The embryonic cells which almost completely fill the body of a 

 pupa are not derived from the muscular nuclei alone ; they are also 

 formed by the proliferation of the cells of the adipose body. This 

 function of the cells of the adipose body was uot previously known. 

 When a larva is on the point of becoming a pupa, numerous 

 daughter cells appear in the midst of their protoplasm ; subsequently 

 the envelope and the nucleus of the cells of the adipose body dis- 

 appear ; the daughter cells are set free, multiply in their turn, and 

 display all the characters of embryonic cells. 



The return of the tissues to the embryonic state is the cause of 

 this very remarkable fact, that at a certain moment the pupa has 

 really the characters of an embryo. When we examine a section 

 made across the abdomen of a pupa of from two to four days stand- 

 ing, we observe that the body is composed of only two layers of 

 central cells, one forming a solid cord, composed of the epithelial 

 cells of the digestive tube which have reverted to the embryonic 

 state, the other peripheral, consisting of the embryonic cells origi- 

 nating from the muscular nuclei and the cells of the adipose body. 



When the tissues of the larva are destroyed, the tissues of the 

 adult form. We know from the investigations of M. Weissraann, 

 that the integuments of the head and thorax are developed at the 

 expense of a certain number of buds preexistent in the larva, and 

 designated histoblasts (Imar/inalscheiben). From not having had 

 recourse to the method of sections, my predecessors have been mis- 

 taken as to the structure of these little bodies ; they are not, as has 

 been supposed, small saccules fiUed with cells. The histoblast, when 

 not much developed, appears in a section to consist of a hollow 

 sphere, one half of which bad been immersed in the other: we may 

 therefore consider it to be formed of two laminae, an internal and 

 an external one. The inner lamina is thick and composed of pyri- 

 form cells placed side by side ; the outer lamina is thin and consists 

 of a single layer of flattened cells. During the development of the 

 histoblast the outer lamina disappears, and the inner lamina in- 

 creases to form the integuments of the adult. The bistoblasts of 

 the eyes present the same structure as the others ; the following 

 are the only peculiarities observed in them. The inner lamina is 

 composed of large cells, very regularly arranged side by side, of a 

 cylindrical form, terminated at its outer extremity by a flattened 

 base, drawn out into a point at the other extremity. Each of them 

 is continuous by its produced extremity with one of the fibrils of 

 * See ' Annals,' ser. 5, vol. vii. p. 352. 



