283 



Finally before the union of the germ nuclei the egg centrosonae grows 

 indistinct and fades into the substance of the surrounding sphere. 



Spheres. Around each centrosome in the early prophase of 

 the first maturation is a faintly staining sphere, the couche cor- 

 ticale of Van Beneden. This grows with the growth of the mitotic 

 figure until in the metaphase it becomes a very large aster, which in 

 the anaphase again becomes more regularly spherical. It is composed 

 of a deeply staining substance, identical with the interfilar substance 

 of the spindle, through which in certain stages the astral rays can 

 be traced to the centrosome (Dia. A.) 



In the second maturation the sphere is formed in the same way 

 as in the first but in the telophase it grows very much larger and 

 partially or entirely surrounds the egg nucleus. Within this sphere 

 the egg centrosome persists for a considerable period as a mass of 

 granules but before the union of the germ nuclei the outlines of the 

 centrosome become indistinguishable and its granules mingle with those 

 of the sphere. After the disappearance of the egg centrosome the 

 sphere loses its radial character and is composed of coarse granules 

 united by a delicate reticulum. 



II. Fertilization. 

 The head and middle piece of th spermatozoon enter the egg 

 during the early prophase of the first maturation; the tail does not 

 enter. The head is transformed into the sperm nucleus and the middle 



l^-PoUtSjimAW; 



A Animal ?o\ä 



^'PoUrßodu 



SfiernN 



C D 



Diagram C. Entrance of sperm, 1st raaturation of Crepidela. 



Diagram D. Formation of sperm nucleus and sphere. 2^ maturation of Crepidula. 



