152 SMALLWOOD. [VOL. II. 



Metamorphosis of tJie Centrosome in Maturation. 



The definiteness and clearness with which the several 

 changes in the centrosome appear in Bulla make these changes 

 the most important of the various stages in maturation and 

 fertilization. In describing the changes of the centrosome, 

 tinder various stages, I have no theoretical points in considera- 

 tion. While the stages figured are clearly differentiated, still 

 there are intermediate stages which graduate imperceptibly 

 into one another. 



In the earliest prophase that I found the central corpuscle 

 was a large solid mass (Fig. 8). Surrounding the central cor- 

 puscle there was a conspicuous area, the cortical zone, which was 

 sharply differentiated from the cytoplasm. The rays are not 

 lost in the cortical zone, as MacFarland has shown for Diaulula, 

 but extend to the central corpuscles, as Lillie has shown for 

 Unio, and Linville for Limnaea. However, I do not find a row 

 of microsomes, as in Unio, limiting the sphere, nor is the bound- 

 ary formed by the fusing of the astral rays, as in Limnaea. 



Second stage (Fig. 9). The central corpuscle has become 

 clearly differentiated into a centrosome. It reacts to stain in 

 a very different manner from what it did in a previous stage. 

 There is now a medullary zone which takes on a plasma stain 

 and is limited by a distinct line. The small dark body in the 

 center is the new central corpuscle. The cortical zone has 

 increased in size and is less easily distinguished from the sur- 

 rounding cytoplasm. The line marking the periphery of the 

 centrosome is the limiting wall of the enlarged central corpus- 

 cle of the previous stage. 



Third stage (Fig. 10). The centrosome has increased in size. 

 The line at the periphery is definite and whole. The central 

 corpuscle of the previous stage has divided into two central 

 corpuscles, which are connected from the first by faint lines. 

 The medullary zone does not take a plasma stain. The corti- 

 cal zone has become very faint and soon disappears as a 

 distinguishable area in the cytoplasm. 



Fourth stage (Figs. 3 and 1 1). The periphery of the centro- 

 some loses its continuity, and openings occur in the wall ; while 



