The maturation and fertilization of the egg of Cerebratulus. 471 



Explanation of Plates. 



These figures, except where otherwise stated, have been drawn 

 with the aid of a camera lucida from sections of the eggs of Cere- 

 hratulus marginatus, at a magnification of 500 diameters, by means of 

 a Zeiss 2 mm apochromatic objective and compensating eye-piece No. 4. 

 Because of the thinness of the sections compared with the size of the 

 eggs, it seldom happened that a single section would show all of the 

 essential features of the egg. In order to render a drawing of such 

 an egg readily intelligible, and in order to avoid the necessity of 

 publishing two or more drawings of a single egg, in a few cases these 

 drawings have been reconstructed in the plates, and several sketches 

 combined into one. Except in those cases where the reconstruction 

 consisted merely in the addition of a single centrosome, or some slight 

 detail, from the very next section the fact has been indicated in the 

 explanation of the figure. And in no case have the structures been 

 foreshortened to any considerable extent. 



Plates 19. 



rig. 1. Germinal vesicle of ripe egg immediately after leaving 

 the body of the worm, and before the appearance of asters. Most of 

 the stain has been extracted from the mass of the chromatin-network, 

 so that only those granules which are to form the chromosomes remain 

 deeply stained. The nucleolus is stained red in the specimen. 



Fig. 2. The first appearance of one of the centrosomes and asters 

 which are to form the polar spindles. A small portion of the nucleus 

 is shown with its iadented margin. 



Fig. o. The minute centrosomes and asters of the later polar 

 spindles are widely separated, and lie in two slight indentations of the 

 germinal vesicle. Five, nearly spherical, chromosomes are shown. The 

 stain has been entirely extracted from the yolk-globules , so that they 

 do not appear in the cytoplasmic reticulum. 



Zool. Jahrb. XII. Abth. f. Morph. 3 J 



