TABLE OF CONTENTS The onion root tip and the whitefish blastula remain the standard introduction to the study of mitosis. The onion has easily observable chromosomes, and the whitefish has one of the clearest views of the spindle apparatus. The testis of the grasshopper and the developing zygote of the roundworm Ascaris are the traditional materials used for viewing the various stages of meiosis.
In a single longitudinal section of a grasshopper testis, one can usually find all of the stages of meiotic development. The stages are also aligned from one pole of the testis. Few other meiotic samples are as convenient. For most materials, meiosis occurs in a more randomly distributed pattern throughout the testis.
Ascaris is utilized to observe the final stages of development in eggs (oogenesis). The Ascaris egg lies dormant until fertilized. It then completes meiosis, forming 2 polar bodies while the sperm nucleus awaits fusion with the female nucleus. When this phenomenon is coupled with the large abundance of eggs in the Ascaris body, it makes an ideal specimen for observing the events of fertilization, polar body formation, fusion of pronuclei, and the subsequent division of the cell (cytokinesis).
The stages of mitosis were originally detailed after careful analysis of fixed cells. More recently, time-lapse photography coupled with phase-contrast microscopy has allowed us to visualize the process in its entirety, revealing a dynamic state of flux.
In early work, so much emphasis was placed on the movement of the chromosomes that the cell...
