228 bulletin: museum of comparative zoology. 



becomes greater (Plate 3, Figure 20). I have never seen for Limnaea 

 the stage shown for Limax in Figure 20. Although every indication of 

 an egg-aster finally disappears, the time of its disappearance is very 

 much delayed as compared with many other animals. 



There are many interesting questions which arise in connection with 

 the disappearance of the egg-centrosome. There are difficulties in proving 

 either that the disappearance is permanent or that it is only temporary. 

 If one maintains that the centrosome after the formation of the sec- 

 ond polar cell simply goes into a resting stage, and thus becomes in- 

 visible, but finally reappears, the only answer that can be made is, 

 that such a statement is an assumption that can neither be proved 

 not disproved. It is an easy matter to find small dense bodies in the 

 region of the egg-nucleus, and even bodies surrounded by radiations. 

 Often many of these small dense bodies may be found in such positions 

 with reference to the two nuclei in the egg as to seem to be significant, 

 but the difficulty comes in deciding which, if any, of these many 

 centrosome-like structures are really centrosomes. For, not only may 

 these structures be found in close relation to the nuclei, but similar 

 appearances are frequent throughout the egg. With such difficulties as 

 these to contend against, it is of the greatest importance that the 

 phases in the regressive metamorphosis of the egg-centrosome be fol- 

 lowed with the closest scrutiny. 



III. The Nucleus. 



In my original plan of work I was little concerned with the nucleus, 

 but as the investigation progressed two problems of great interest and 

 importance claimed my attention. These are : first, the relation of the 

 nucleoli or karyosomes to the chromatin in the resting nuclei, and, 

 secondly, the question of the reduction division in the Roux-Weismann 

 sense ; that is to say, a reduction of qualities by a transverse division of 

 the single chromosomes in the second maturation spindle. I shall take 

 up the second of these questions first, because it comes first in the 

 stages of development which I have studied. 



1. Division of Chromosomes. 



The apparent variation in the number, size, and form of the chromo- 

 somes in the maturation spindles of Limax maximus made the study of 

 these elements a particularly difficult and perplexing one. As long as I 

 worked exclusively with this material, the possibility of ever being able 



