MUSEUM OF COMPARATIVE ZOOLOGY. 143 



It is an interesting fact, that even the most thoroughly degenerated 

 membranes have numerous nuclei in all stages of division. The divid- 

 ing nuclei have undergone the same degenerative alteration as the rest. 

 It is impossible to state whether these nuclei had begun to divide after 

 the regressive change, or had been overtaken by these changes while 

 undergoing division ; and it is equally impossible to say whether 

 degeneration would have prevented the nuclei from completing their 

 division. The division is essentially like that of younger nuclei, but 

 often unsymmetrical. 



Not all the degenerative changes are confined to the nuclei. The 

 cells also give evidence of modification. Their walls become more 

 distinct, not only because they are denser and thicker, but on account 

 of their stainability with hsematoxylin. The cytoplasm frequently has 

 a reticulated structure, which is densest about the nucleus. In the 

 oldest membranes, certain large groups of cells have nuclei surrounded 

 by a narrow bright ring, and outside this a much broader halo of a 

 radiating structure, which takes a deeper stain than the rest of the 

 cytoplasm (see Fig. 34). The appearance of the whole is strikingly 

 like that of the " attraction spheres " of ovarian and other cells, but in 

 this case has certainly nothing to do with mitosis. If the cell contains 

 two nuclei, or a dividing nucleus, each daughter nucleus is surrounded 

 by a halo. In early stages of division, however, the elongated nucleus 

 has a single halo. I am unable to account for these appearances ; I do 

 not regard them as attraction spheres, but rather as a result of degener- 

 ation. The attraction sphere should radiate from a centrosome ; here 

 it radiates from the nucleus as a centre. I may state, in passing, that 

 my search for centrosomes in the serosa has been wholly unsuccessful. 

 The pale ring is very generally present around nuclei that have under- 

 gone degeneration. It seems to have no intimate connection with the 

 radiating zone, being frequently found where the latter is absent. 



The life history of the serosa cells corresponds closely with that of 

 certain cells in the Malpighian vessels of Aphrophora spnmaria de- 

 scribed by Carnoy ('85, p. 219). The cells at the two extremities of 

 the tubes contain nuclei not gi-eatly different from those of young 

 serosse, but the nuclei of the middle portion are irregular, jagged, 

 and filled with amorphous chromatin. They therefore bear a strong 

 resemblance to the degenerated nuclei of the serosa. Furthermore, the 

 origin of the peculiar nuclei of the middle portion of the Malpighian 

 vessel agrees closely with that of the degenerated nuclei of an old 

 serosa. It is thus described by Carnoy (p. 220) : '^Sur les petites 



