394 MEAD. 
invaginates in the vicinity of each aster in the manner already 
described by several authors.! It is afterwards resolved into 
the cytoreticulum, though the huge nucleolus and other features 
of the nucleus do not immediately disappear. 
Definite chromosomes are seen to be attached to the achro- 
matic fibrils of the two asters, and to take their position in the 
equatorial plate in the ordinary way. The centrosomes become 
doubled and are demonstrable in each aster as two exquisitely 
clear dark dots in the midst of the light yellow centrosphere. 
The spindle thus formed swings around to occupy a radial 
position at the periphery of the egg, and remains in the 
metaphase until the sperm enters. On the entrance of the 
sperm the karyokinetic activity is resumed and the maturation 
processes are completed.2 Usually, before fertilization, the 
nucleolus of the germinal vesicle breaks up and disappears ; 
the last traces of it are seen in the neighborhood of the 
maturation spindle. 
The foregoing observations convince me that the asters and 
centrosomes in the Chetopterus ovum arise by a modification 
of the cytoplasmic reticulum. The phenomena of their origin 
and their relation to the secondary asters are similar to those 
described by Reinke? in the tissue cells of the larval salamander. 
Watasé says he has “seen in the egg of Macrobdella a series of 
thirteen asters, ranging from a diminutive aster with a microsome 
for its center to the normal aster with a veritable centrosome.” # 
The fact that in Chetopterus the secondary and primary 
asters were formed when the eggs were transferred from the 
body-cavity of the worm into the sea-water, suggests a compar- 
ison of these phenomena with the production of ‘artificial 
centrospheres’”’ in sea-urchin eggs by adding more salt.® 
1 Compare Wheeler: Myzostoma. Journ. of Morph. vol. X, No.1. Griffin: 
Thalassema. TZyvans. V. Y. Acad. Sct., June 2, 1896. 
2 Mead: Joc. cit. 
3 Reinke: Joc. cit. 
4 Watasé: Origin of the Centrosome. iol. Lectures, Marine Biological 
Laboratory, 1894, p. 285. 
5 Morgan: The Production of Artificial Centrospheres. Archiv f. Entwickel- 
ungsmechanik, Bd. III, Heft 3. 
