Plate I. 



First Maturation Division of Crepidula plana. 



Fig. 1. — Ovarian egg showing germinal vesicle and spot, the latter composed of two parts. 



Fig. 2. — Unfertilized egg from the uterus ; optical section showing what appear to be tw ; o 

 centrosomes lying next the nucleus. Obj. 3 mm. , Occ. 4 (Zeiss Apochromat). 



Figs. 3 and 4. — Fertilized eggs from the uterus showing indentation of the nuclear mem- 

 brane, limn network and growth of chromosomes. Obj. 7 \ (Leitz), Occ. 4. 



Figs. 5 7. — Fertilized eggs just laid ; formation of spindle and elimination of chromatin 

 and nucleolus. 



Fig. 8. — Prophase of first maturation ; chromosomes of various shapes, centrosomes 

 irregular in outline. 



Fig. Sa. — Tangential section through centrcsome and sphere of a stage similar to Fig. 8. 



Figs. 9 and 10. — Early metaphase showing two characteristic positions of spindle. Obj. 3 

 mm., Occ. 4. 



Fig. 11. — Same as preceding, showing cross-shaped chromosomes and irregular centro- 

 somes with light center. 



Fig. 12. — A characteristic metaphase ; chromosomes accurately drawn both as to form and 

 position ; the chromosome Ch\ is displaced out of spindle, probably by the knife. Centrosomes 

 show clear center. 



Fig. 12a. — Cross section through the equator of a spindle of the stage of preceeding. 

 Processes of spindle substance radiate into the cytoplasm. Many chromosomes are surrounded 

 by a linin sheath. 



Fig. 13. — Early metakinesis ; the daughter chromosomes are connected by linin threads 

 which in some cases are moniliform. The centrosomes are hollow and the spheres more regular 

 than in preceding stages. 



Fig. 14. — Anaphase ; interfilar substance of spindle between chromosomes and spheres ; 

 chromosomes with faintly staining centers ; centrosomes contain a hollow central corpuscle ; periph- 

 erel centrosonie and sphere flattened against the egg membrane. 



Fig. 15. — The chromosomes lie at the borders of the spheres ; centrosomes and central 

 corpuscles elliptical ; protrusion of outer pole of spindle. 



Fig. 16a. — Connective fibres granular; central corpuscle transformed into amphiaster 

 (netrum) of second maturation division ; outer pole of spindle not protruding. 



Fig. 10. — Slightly later stage than preceeding; chromosomes pass into spheres; centro- 

 somes and inclosed amphiasters (netra) at both poles ; protrusion of first polar body. 



Fig. 17. — Mature spermatozoon of C. plana. 



Fig. 18. — Sperm head immediately after entering the egg from a preparation of the same- 

 stage as Fig. 3. 



Fig. 19. — Rotation of sperm head ; appearance of middle piece granules ; egg in stage of 

 Fig. 4. 



Fig. 20. — Shortening of sperm head ; egg in stage of Figs. 5-7. 



Fig. 21. — Formation of sperm nucleus ; egg in stage of Figs. 10-14. 



