. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . FIG. 152.—Egg of the worm, Myzo-stoma glabrum, being fertilized byspermatozoon. (After Wheeler.) 268 EVOLUTION AND ANIMAL LIFE. FIG. 153.—Diagrams illustrating the fertilization of the egg: A, Egg surrounded byspermatozoa; on the right, one has just penetrated the egg membranes and is en-tering the egg cytoplasm; egg nucleus in the center. B, Egg nucleus with chromatinreticulum on the left; on the right, the sperm nucleus preceded by its centrosomeand att
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . FIG. 152.—Egg of the worm, Myzo-stoma glabrum, being fertilized byspermatozoon. (After Wheeler.) 268 EVOLUTION AND ANIMAL LIFE. FIG. 153.—Diagrams illustrating the fertilization of the egg: A, Egg surrounded byspermatozoa; on the right, one has just penetrated the egg membranes and is en-tering the egg cytoplasm; egg nucleus in the center. B, Egg nucleus with chromatinreticulum on the left; on the right, the sperm nucleus preceded by its centrosomeand attraction sphere. C, Egg nucleus on the left, sperm nucleus on the right ofthe center of egg; aster fibrils preceding the fission of the centrosome. D, Thecentrosome has divided, the two attraction spheres separate to form the firstcleavage of the spindle; the chromosomes of the egg and sperm nuclei clearly visibleand distinguishable (in the figure the egg chromosomes are black, the egg spermchromosomes shaded). E, First cleavage, showing spindle with splitting chromo-somes. F, Completion of first cleavage, two-celled stage, each nucleus showing fourchromosomes, two from the egg and two from the sperm. (After Boveri.) FACTORS IN ONTOGENY 269 nucleus being slig
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