214 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



from a study of two species of Lirrmsea. The results on Limnsea to 

 some extent repeat the most excellent work by Kostanecki und Wier- 

 zejski ('96) on Physa, a near relative of Limnsea. However, additional 

 points of interest in regard to chemical phases in the centrosome, to- 

 gether with some facts concerning reduction division in the second 

 maturation spindle, make this something more than a mere repetition 

 of their work. 



Since the publication of Professor Mark's monograph on Limax 

 campestris, in 1881, the technique of embryology has been so much 

 developed that a re-investigation of the maturation and fertilization of 

 this genus of pulmouates with special attention to the history of the 

 centrosome seemed likely to yield interesting and important results. 

 The incompleteness of my series of stages in Limax material will, how- 

 ever, necessitate further work on this genus, which I hope to complete 

 at some time in the near future. 



By each addition to the already great mass of cytological litera- 

 ture, it becomes more apparent that if the centrosome is a perma- 

 nent organ of the cell, it is an organ which goes through various and 

 complicated phases. Certain facts impel us to believe that there 

 is a variation in the chemical condition of the centrosome. For 

 example, in some preparations, at a certain stage of maturation, 

 there is no centrosome (nor centriole) to be distinguished as a 

 deeply staining body at the poles of a perfectly formed spindle, whereas 

 in other preparations at apparently the same stage and treated in the 

 same manner, one may observe centrosomes which are deeply stained and 

 of enormous size in comparison with the volume of the cell. This great 

 variation of the centrosome is, however, not inconsistent with the idea 

 of its permanence in the first maturation spindle, where the astral rays 

 always indicate its presence. When, however, no astral rays are visible, 

 the difficulties in the way of identifying the centrosome are exceedingly 

 great, so great in fact as to render it well-nigh impossible to distinguish 

 it from the yolk granules. 



Collection of Material. 



I have collected and preserved eggs of Limax agrestis, Limax maximus, 

 and of two species of Limnsea. Limax maximus lays in the vicinity of 

 Cambridge, Mass., during October, November, and December. Limax 

 agrestis begins laying in early summer, and may, under especially favor- 

 able circumstances, continue to lay late into the winter. The eggs of 



