. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. Fig. 74. Triaster and tetraster mitotic figures from a sea-urchin, Strongylocenlrottis lividus. (After Baltzer, Verh. Phys. Med. Gesells, Wurzburg, igoS.) that two spermatozoa enter the egg instead of one. By increasing the concentration of sperm, the percentage of such dispermic (or poly- spermic) eggs can be enormously increased. Thus in two parallel experi- ments, eggs placed in water with only a few spermatozoa resulted in a hundred normal monospermic and no di- or polyspermic fertilizations. On the other hand, eg
. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. Fig. 74. Triaster and tetraster mitotic figures from a sea-urchin, Strongylocenlrottis lividus. (After Baltzer, Verh. Phys. Med. Gesells, Wurzburg, igoS.) that two spermatozoa enter the egg instead of one. By increasing the concentration of sperm, the percentage of such dispermic (or poly- spermic) eggs can be enormously increased. Thus in two parallel experi- ments, eggs placed in water with only a few spermatozoa resulted in a hundred normal monospermic and no di- or polyspermic fertilizations. On the other hand, eggs placed in very concentrated sperm gave only eleven monospermic and eighty-nine di- or polyspermic fertilizations. When an Echinoid egg is fertilized by two spermatozoa both sperm nuclei (typically) fuse with the egg nucleus, and the centrosome intro- duced by each spermatozoon divides as if it were the only one—hence we get a zygote nucleus with 3^ (= 54) chromosomes and four centro- somes. A four-pole spindle figure is then produced {tetraster), and at the first division the egg divides simultaneously into four blastomeres instead of into two. In the most usual type of tetraster (and the only type which we will. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Agar, Wilfred Eade, 1882-. London Macmillan
Size: 2487px × 1004px
Photo credit: © Central Historic Books / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectcells, bookyear1920
