. The cell in development and inheritance. Cells. THE 307 the changes of mitosis, the chromatin-network giving rise to a group of chromosomes and a spindle, or more frequently a fan-shaped half-spindle, arising from the achromatic substance. In some cases not only a complete spindle appeared but also asters at the poles, though no centrosomes were observed (Fig. 151). Morgan's experi- ments along the same lines were mainly performed upon the sea- urchin Arbacia, but included also the eggs of Asterias, Siptincnlus, and Ccrchmtitlns (Figs. 150, 151). In these eggs numerous asters may
. The cell in development and inheritance. Cells. THE 307 the changes of mitosis, the chromatin-network giving rise to a group of chromosomes and a spindle, or more frequently a fan-shaped half-spindle, arising from the achromatic substance. In some cases not only a complete spindle appeared but also asters at the poles, though no centrosomes were observed (Fig. 151). Morgan's experi- ments along the same lines were mainly performed upon the sea- urchin Arbacia, but included also the eggs of Asterias, Siptincnlus, and Ccrchmtitlns (Figs. 150, 151). In these eggs numerous asters may arise in the cytoplasm, if they are allowed to lie some time in sea- »m^ ^^r^-rr-'^- . \ \'A\ .\. , /^ '.y^< f : /v ^ •MVJ// A. rf\' /i"i Fig. 150.— Formation de novo (?) of centrosomes. [.4, B, MEAD; C, MORGAN.] A. Unfertilized egg of Chivfoptei-us with " secondary asters " developed a few minutes after the egg is placed in sea-water. B. Slightly later stage with two definitive polar asters and centrosomes. C. Large "sun" (transformed polar aster) containing numerous small "secondary asters " and centrosomes, from unfertilized egg of Ceiebratulus after 22 hours in % sodium chloride solution. water or treated by weak solutions of sodium or magnesium chloride. These asters often contain deeply staining, central granules indistin- guishable from the centrosomes of the normal asters ; and, what is of high interest, such of them as lie near the nucleus take part in the irregular nuclear division that ensues, forming centres toward which the chromosomes pass. These divisions continue for some time, the chromosomes being irregularly distributed through the ^gg, and giving rise to nuclei of various sizes apparently dependent upon the number of chromosomes each receives. After a variable number of such. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration a
Size: 1799px × 1389px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1906
