Plate III. 



Fecundation and First Cleavage of C. plana 



Figs. 42 and 43. — Approach of sperm nucleus and sphere to those of egg ; one accessory 

 asterin each egg ; one large accessory aster present. Obj. 3 mm., Occ. 4. 



Fig. 44.— Union of egg sphere and sperm sphere ; contact of germ nuclei. Obj. 3 mm., 

 Occ. 4. 



Fig. 45. — Germ nuclei in contact; spheres still distinct; yolk granules between the two 

 spheres and nuclei. 



Fig. 46. — Egg and sperm spheres fused and yolk granules inclosed. Obj. 3 mm., Occ. 4. 



Fig. 47. — The fused spheres surround the germ nuclei; cleavage centrosomes appear at 

 border of spheres. Obj. 3 mm., Occ. 4. 



Fig. 48. — Sphere substance no longer visible ; one centrosome and sphere in connection 

 with each germ nucleus ; same stage as Fig. 53. Obj. 3 mm., Occ. 4. 



Fig. 49.— Germ nuclei surrounded by the fused spheres. 



Figs. 50 and 51. — Similar to preceding ; cleavage centrosomes appear at the border of 

 the fused spheres ; chromatin granules vesicular. 



Fig. 52. — Cleavage centrosomes at the poles of the germ nuclei ; no central spindle ; 

 chromatin granules vesicular. Obj. T \ (Leitz), Occ. 4. 



Fig. 53. — One -centrosome and half spindle in connection with each germ nucleus ; nuclear 

 membrane indented opposite centrosomes ; chromosomes forming out of granules, other granules 

 dissolving in nuclear sap. 



Fig. 54. — -First appearance of central spindle between the two cleavage centrosomes ; 

 chromosomes aggregating in the spindle ; other granules dissolving ; spheres increasing in size and 

 substance radiating from them through the cytoplasm. 



Fig. 55. — Prophase of first cleavage ; oxychromatin granules arranged along the spindle 

 fibres. Obj. T V (Leitz), Occ. 4. 



Fig. 56. — Metaphase of first cleavage ; "heterotypic" division of the chromosomes. 



Fig. 57. — Similar to preceding ; centrosomes slightly hollow. 



Fig. 58. — Anaphase of first cleavage ; centrosomes hollow ; spheres alveolar. 



Fig. 59. — Late anaphase ; chromosomal vesicles closely appressed to sphere ; centrosomes 

 and spheres filled with a delicate reticulum. 



Fig. 60. — Telophase of first cleavage ; beginning of the bending of the spindle axis : nucleus 

 in the left cell partly divided into two portions of which the upper half is from the egg nucleus and 

 the lower from the sperm. 



Fig. 61. — Resting stage at close of first cleavage ; spindle axis bent on itself through nearly 

 180°; nuclei almost in contact with each other, centrosomes and spheres near surface and mid- 

 body carried down to the middle of the egg. The mid-body is a hollow sphere, like a centrosome, 

 surrounded by a dark area, like a sphere. 



