No. 2.] ORGANIZATION OF THE EGG OF UNIO. 285 



DESCRIPTION OF PLATES. 



All figures are from camera drawings of sections of the eggs of Unio compla- 

 nata. Most of the figures are from single sections, but when two or more sec- 

 tions are combined in a single drawing this is always stated in the description. 

 The lenses used for the drawings were Zeiss Apochromat 2 mm. Homog. Immers. 

 and compensating ocular No. 6, except where otherwise stated. The figures are 

 all drawn from eggs killed in Boveri's picro-acetic mixture, and stained on the 

 slide with Heidenhain's iron-haematoxylin, followed or preceded by Bordeaux 

 red in aqueous solution. Needless to say other methods were used, but the 

 drawings are not published. The sections were 5 or 6 2/3 ju in thickness. 



EXPLANATION OF PLATE XXIV. 



Fig. I. Radial section through the egg. Early stage of the first maturation 

 spindle. Sperm-head and comet-like sperm-amphiaster. 



Fig. 2. Perfect axial section. Part of the egg-membrane with the micropyle 

 is shown. Somewhat later stage of the first maturation spindle than in Fig. i. 

 The entire path of the spermatozoon is shown. Peripheral distribution of yolk- 

 granules. The section was injured before the completion of the drawing, so the 

 maturation spindle is represented somewhat diagrammatically. 



Fig. 3. Sperm-nucleus in the same position as in Figs, i and 2 ; but the sperm- 

 centrosome is just dividing. 



Fig. 4. Sperm-head and amphiaster from a stage of maturation similar to 

 Fig. 2. The plane of separation of the sperm-centrosomes is nearly at right 

 angles to the penetration path. Many similar eggs found. The distal sperm- 

 centrosome is divided in three parts. 



Fig. 5. Turning of the first maturation spindle ; stage intermediate between 

 Figs. I and 2. Combination of two successive sections. 



Fig. 6. Section of unfertilized egg with first maturation spindle. In the other 

 eggs of the same lot, which were fertilized, the first polar globule was already 

 fully formed, and the second in process of formation. 



Figs. 7 a and 7 b. Sperm-nucleus and sperm-aster from different sections of the 

 same egg, in a stage of maturation intermediate between Figs. 2 and 5. The cen- 

 trosome in 7 a is attached by a fiber to the tip of the sperm-nucleus, and the aster 

 is gone. The aster around the centrosome in 7 (5 is already breaking up. 



Fig. 8. Slightly later stage of degeneration of the sperm-asters. But one cen- 

 trosome could be found ; the one that originally lay nearer to the sperm-nucleus 

 is entirely indistinguishable. 



Fig. 9. Radial section of the first maturation spindle at the time of the meta- 

 phase ; one of the chromosomes has already divided transversely. Part of the 

 inner aster was drawn from another egg, where it showed more plainly. , 



Fig. 9 a. Sperm-head in the same egg from which Fig. 9 was drawn. The 

 sperm-centrosomes are now no longer distinguishable ; and the clear area has 



