A cytological study of the egg of Cumingia with 
special reference to the history of the chromosomes 
and the centrosome. 
By 
H. E. Joi’dan, Ph. D. 
Associate Professor of Anatomy, University of Tirginia. 
With plates XVI— XVIII. 
The material of this investigation , ovarian and free fertilized 
eggs of the lamellibranch mollusc, Cumingia tdlinoides (Coxrad), 
was collected at Woods Hole, Mass. during the summer of 1908. The 
preservatives used were sublimate-acetic and picro-acetic mixtures, 
the former giving the better results. Sections were cut at 8 microns, 
stained with iron-hematoxylin with and without counterstain and 
studied with a B. and L. 1./12. oil immersion lens and no. 1 ocular. 
All drawings were made with this combination and a camera lucida. 
Einer details were studied by aid of a Zeiss 2 m. m. apochromatic 
oil immersion lens and added free-hand. 
The egg of Cumingia is peculiarly ‘ favorable for experimental 
embryoiogical research and promises to be more extensively employed 
in future work. For this reason it seems desirable that both the 
oögenesis and the cell lineage should be described. This paper 
represents an attempt to do the former as far as the nature of the 
material yields definite results 0- 
b A preliminary report giving the main outlines of the oögenesis was 
made before the American Society of Zoologists at the Baltimore meeting in 
1908 — see Science for March 12, 1909. 
Archiv f. Zellforschnng. IV. 
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