174 TRANSACTIONS OF THE [may 11, 
centrosome in the first cleavage figure. Von Rath (8) also de- 
scribes a similar state of affairs in the Echinoderms. 
Both v. Beneden and Boveri have given detailed descriptions of 
the morphology of the polar systems, centrosome, sphere, rays, 
etc. Between the accounts of these two investigators there ex- 
ists considerable discrepancy which has been carefully considered 
by Hacker (4). In Thalassma it would seem that morphologi- 
cally the minute dark staining granule is equivalent to Boveri’s 
“centriole,” the cloudy surrounding area to his “ centrosom,”’ 
the centrosphere to the “ hellem Hof,” and the crown of astral _ 
rays to his ‘‘archoplasm.” Furthermore, my “ centrosphere ” 
seems to correspond to the centrosphere described by Stras- 
burger in Vegetal Karyokinesis. Functionally, however, the 
black granule (“ centriole ”’) is here the true centrosome, since it 
alone of all the elements of the polar system persists and multi- 
plies by division. We have seen that the rays centrosphere 
and cloudy area, which are in turn differentiated about the cen- 
trosome, are formed only during the stages immediately pre- 
ceding the establishment of the chromosomes and equatorial 
plate, and commence to break down the moment the anaphase is 
fairly under way. They subsequently degenerate in situ and 
disappear completely, while the centrosome, unaccompanied by 
any surrounding archoplasm mass, migrates to the periphery of 
the centrosphere, where it sets up a newseries. From this fact, 
as well as from the extreme minuteness and constancy in size of 
the centrosome, it would appear that the rays, centrosphere and 
cloudy area are probably to be looked upon as differentiations 
of preéxisting cytoplasmic material, or possibly the product of 
a specific form of metabolic activity set up in the cytoplasm by 
the centrosome, which disengages the forces at work in mitosis. 
ZOOLOGICAL LABORATORY CoLUMBIA UNIVERSITY, 
May, 1896. 
[Since the above went to press there has appeared an ex- 
tremely interesting article by Kostanechi and Wierzejshi (Ueber 
das Verhalten der sogen. achromatische Substanzen im befruch- 
teten Ei. Nach Beobachtungen an Physa fontanalis. Archiv. f. 
Micr. Anat. Bd. 47 H. 2 April, 1896). From the figures and ac- 
counts of these authors it appears that Physa very closely re- 
sembles Thalassema. In the precocious division of the centro- 
some, the origin of the second polar spindle, the absence of a 
quadrille, the division of the second polar centrosome previous 
to its disappearance, the formation of the sperm amphiaster, and 
in some cases the curving of the rays—in all these points the 
