270 PROCEEDINGS OF THE ACADEMY OF [^larcll, 



determined; and there is the surety in such study that the chromo- 

 somes are relatively large structures, which exist in fact and are 

 not artificially produced by the mode of preparation necessary for 

 their study. The chromosomes are not apparently formed de 

 novo — at least there is as yet no good proof in any case that they 

 are so formed; while, on the contrary, there is a considerable 

 amount of evidence to show that they are structures which persist 

 from generation to generation, even though this is a persistence 

 nvolving a great amount of metabolic change. Astral radiations 

 appear and disappear, or at least disappear as radiations ; nucleoli 

 are apparently accumulations of metabolic substances of no 

 morphological regularity, as I have shown in another place;' and 

 recent experimental studies would show, though perhaps in contra- 

 diction to the anatomical studies, that the centrosomes may be 

 formed anew. But the chromosomes show more fully than any 

 of these cellular structures a certain degree of morphological 

 stability, and this fact, taken in connection with their greater 

 adaptability for study, entitles them to a basic place in the study 

 of the cell's evolution, as well as in the study of evolution in 

 general. 



EXPLANATION OF PLATE X. 



All figures liave been drawn to the same scale with the camera lucida at 

 the level of the base of the microscope, with the Zeiss homogeneous immer- 

 sion yV, ocular 4, tube length 180 mm. 



The bounding line in figs. 1-4, 6-10, 14-18, and 20-23 represents the cell 

 membrane ; in figs. 5, 11-13, and 19, the nuclear membrane. In lateral 

 views of the mitotic spindles (figs. 2, 4, 7, 15, IG, ^1) the mantle fibres are 

 the only achromatic elements shown, and are represented thicker than they 

 are in reality. 



Plate X, fig. 1. — Tingis clavata, pole view of monaster stage of the 

 first maturation mitosis. 



Fig. 2. — Idem, lateral view of the same stage. 



Fig. 3. — Corixa verticaiis, pole view of monaster stage of the first mat- 

 uration mitosis. 



Fig. 4. — Idem; lateral view of the same stage. 



Fig. 5. — Idern, nucleus in post-synapsis stage. 



P^'ig. G. — Cymus luridus, pole view of monaster stage of the first matura- 

 tion mitosis. 



Fig. 7. — Idem, lateral view of the same stage. 



Figs. 8, 9. — Lygus pratensis var., pole views of monaster stage of first 

 maturation mitosis. 



Fig. 10. — Idem, pole view of monaster stage of second maturation mitosis. 



Fig. 11. — J^^'abis aimulatus, nucleus in growth period (late telaphase), 



» Journal of Morphology, Vol. XV, 1898. 



