POLARITY AND BILATERALITY OF THE ANNELID EGG. 67 



zation of the ground substance. The basophile granules pre- 

 dominate and are arranged in a loose spongy network. This area 

 is bounded by the spindle area on the animal pole side, and by 

 the spherular zone (3) on the other sides. The boundary towards 

 the spindle area is a little indefinite, fairly definite on the other 

 sides. Its form is that of an indented sphere, the indentation 

 being produced by the spindle area. Its position is about the 

 same as that of the germinal vesicle before rupture. 



Together, the spindle area and the central spongy area form 

 an ovoidal mass, the small end of which reaches the surface at 

 the animal pole, while the large end extends below the center of 

 the egg. 



The other zones of the egg are arranged concentrically to this 

 mass, and are therefore open at the animal pole and thickest at 

 the sides and vegetative end. 



3. The spherular zone is so named because it contains the major 

 part of the endoplasmic spherules or endoplasmic acidophile 

 granules (yolk). In section, it appears as a very deep gourd- 

 shaped crescent. The thin horns reach up above the level of the 

 equatorial plate of the spindle. 



4. The intermediate spongy zone lies between (4) and (5). In 

 many eggs it is barely noticeable, in others it is very prominent. 

 Like the central spongy zone, it is not a vacuolated portion of 

 the ground substance, but it is characterized by paucity of gran- 

 ules, and so appears unstained in sections. It is a genuine struc- 

 tural feature as is proved by experiments, and also by the fact 

 that it becomes more pronounced in eggs allowed to remain in 

 the sea water (Fig. 5). 



5. The ectoplasmic zone is the peripheral boundary of the egg, 

 except that it is defective at the animal pole where the spindle 

 area comes to the surface. It is characterized by the presence 

 of spherules that differ in certain properties from those of the 

 spherular zone (endoplasm), as described in detail in a previous 

 paper. 



In eggs allowed to stand in sea water without fertilization 

 these areas become more strongly differentiated (Fig. 5), arid the 

 fact that the egg has a concentric as well as a polar organization 

 stands out more strongly. The central spongy area now appears 



