9 o 



WM. S. MARSHALL. 



embryos, specimens but a few days old, insects one third full 

 grown and mature insects. 



In a longitudinal section through an embryo two or three weeks 

 before it was ready to emerge from the egg, the Malpighian 

 tubules were easily seen and the condition of the nuclei noted. . The 

 tubules were narrow and short but, proportional to the length of 

 the body of the insect, were, I judge, about the same as in mature 

 specimens. In these tubules a few mitotic figures were seen 

 (Fig. i) but no instance of a direct nuclear division could be 

 found. It was impossible to find any cell boundaries and nothing 

 could, at this stage, be determined as to the binucleate character 

 of the cells. In the tubules of mature insects the nuclei often 

 lie in pairs, the two of each pair being nearer to each other than 

 to the others ; each pair being within a single cell. In the em- 

 bryos studied the nuclei were very much crowded together and 

 no such arrangement was possible ; this crowding together of the 

 nuclei made the relationship in size of nucleus to cell very dif- 

 ferent from what was found in the mature insect where the cell 

 was, in proportion to the size of its nuclei, very much greater 

 than in the embryo. 



In the tubules of very young walking-sticks, four to six days 

 old, no dividing nuclei were seen. Both in structure and in rela- 

 tive size proportional to the tubule, or rather that part one might 

 imagine to be the cell, the nuclei were here similar to those found 

 in the embryo. Here and there two nuclei were seen with their 

 opposing surfaces very close to or touching each other — if 

 cell boundaries had been visible these two nuclei would, no doubt, 

 have been within the same cell. In these young insects it was 

 noticed that other organs did not show nearly so many mitotic 

 figures as were found in maturer specimens. 



In insects about one third grown the cells of the tubules were, 

 in proportion to the nuclei, much larger than in the younger in- 

 sects. In the old specimens, none of which had completed egg- 

 laying and some had not yet begun, no mitotic figures were ever 

 seen although, as already mentioned, nuclei dividing indirectly 

 were fairly abundant in the other organs of the body. Thou- 

 sands of cells were examined from the older insects and no trace 

 of mitosis was ever seen in the Malpighian tubules. 



