Centrosome in Enucleated Egg-Fragments. 289 



results; in tact, cytasters appeared in almost all cases. In sec- 

 tions of the enucleated fragments, in which asters were produced, 

 I found that in all the cytasters the centrioles were present. 



The egg of Cerebratulus is better suited to our present purpose 

 than that of sea-urchin. The egg, when removed, has a large 

 germinal vesicle. In some twenty minutes the nuclear membrane 

 fades away, and in an hour or so the egg being still unfertilized, 

 the first maturation mitosis reaches the metaphase. The mitosis, 

 however, does not proceed beyond this stage, unless fertilization 

 takes place. ^ One can, therefore, in the nemertine egg trace the 

 progressive changes of condition in the cytoplasm from the primary 

 oocyte onward. Moreover, the asters of Cerebratulus have very 

 well defined centrioles, which seem to have, even in the division 

 stages, far greater power to resist the action of various fixing fluids 

 than those of the sea-urchin egg. 



To avoid confusion I shall follow strictly Boveri's definitions 

 of centriole, centrosome and centroplasm, using the term "aster" 

 for the whole structure including the ray system, archiplasm, 

 centroplasm and centriole. The aster without the centriole (if 

 such aster exist) I shall call "pseudaster" (= Boveri's pseudo- 

 sphere). In accordance with Wilson I use the term "cytaster" 

 for an aster or pseudaster which is unconnected with nuclear 

 matter. 



II. SOLUTIONS AND PRECAUTIONS AGAINST ACCIDENTAL 

 FERTILIZATION. 



The means of producing cytasters that were tried were: shaking, 

 ether, MgCU, CaCl, KCl, and NaCl. Of these the first two di^d 

 not cause any perceptible change on the eggs, while the other four 

 modified mitoses and produced the cytasters in various degrees. 

 The following solution of CaCU proved best of all; it was, there- 

 fore, used almost exclusively for the experiments on enucleated 

 fragments : 



^I did not succeed in producing the normal polar bodies artificially. Although, as a matter of fact, 

 in a small percentage of the CaCl2 eggs and the CaCl2+KCl eggs (the latter being Professor Wilson's 

 material} one or two polar bodies were extruded, yet in these cases the'mode of maturation was so abnor- 

 mal that it was thought undesirable to use the eggs thus matured for other purposes. 



