Some Observations on the Chromosome Vesicles in the Maturation ete. 99 
others. In the metaphase and anaphase no definite sphere substance 
is noticed, it is again evident in the early prophase of the second 
maturation (Figs. 8, 9, 10), a slight trace of it is found in Fig. 14, 
but there is no marked evidence of its presence in Figs. 12 and 13. 
In a few instances in the telophase of the second maturation, the 
nuclear vesicles are surrounded by a clear area (Figs. 17, 19, 21), 
this I take to be the sphere substance of ConkLin and others. 
The first evidence in the centrosome of the prophase stage of 
the second maturation is the division of the centriole into two equi- 
valent parts. At first they lie elose together (Fig. 7), but soon 
separate, remaining connected by more or less regular strands which 
take a basie stain (Fig. 8). These two centrioles with the potential 
spindle uniting them give rise in a large measure to the second 
maturation figure. With the inerease in size of the centrosome, the 
centroplasm creases to take a plasma stain, soon breaks appear in 
the wall and astral rays form about the centrioles.. The different 
positions in the centroplasm which the forming spindle assumes de- 
monstrate beyond a doubt that in these Nudibranchs the old wall 
does not contribute to the formation of the new centrosomes as is 
held by MacFartLanD (97) but the formation of the spindle does 
agree with similar changes in Orepidula and Haminea. While there 
is no partieular rest period or division into distinet stages in this 
metamorphosis of the centrosome, for convenience in description, we 
may now apply the term centrosome to the bodies previously de- 
signated as centrioles although still surrounded by the centroplasm 
of the old centrosome. The fibres that unite the two new centro- 
somes become more prominent and longer. The wall of the old 
centrosome breaks up into smaller and smaller pieces until it be- 
comes indistinguishable from the granules in the eytoplasm; the 
centroplasm likewise disappears. The old astral fibres and rays do 
not persist as such and become a part of the second maturation 
figure. The new spindle has more prominent fibres on the periphery 
than within, the central ones frequently branching an anastomosing. 
In Fig. 9, the two centrosomes are each a single body; while 
in a later stage, each is composed of centriole and centroplasm, 
these parts being derived as already described in the first maturation. 
The further history of the centrosome in the second maturation figure 
is the same as in other mollusea, disappearing entirely as the chro- 
mosomes become transformed into the female pronucleus. 
The further changes in the spindle are parallelled so far as I 
7* 
