188 INTRODUCTION TO CYTOLOGY 



spores. Any fibers which these furrows encounter as they grow inward 

 are' probably incorporated in the new wall, but they play no prominent 

 part in wall formation: the development of the furrows appears to be 

 entirely independent of the fibers present. 



In his first paper (1916) Farr states that the microspore tetrads of 

 the bilateral type are usually formed by the cell plate method, a wall 

 being formed across the diameter of the microsporocyte on the connecting 

 fibers after the first maturation mitosis, and the two daughter cells 

 being divided in a similar way after the second mitosis. In his second 

 contribution (1918) he shows that in Magnolia such tetrads also are 

 formed by furrowing. After the first mitosis a cleavage furrow starts 

 -to form, but its development is arrested until after the second mitosis, 

 when it resumes its growth toward the center and forms a wall across the 

 diameter of the spherical protoplast. At the same time other new 

 furrows subdivide each hemisphere, so that four uninucleate microspores 

 result. Farr states that no case of bipartition by furrowing is known in 

 the higher plants; bipartition begins in Magnolia, but the furrow ceases to 

 grow until other furrows are formed after the second mitosis, the eventual 

 division occurring by quadripartition. In the lower plants, however, 

 bilateral tetrads may be formed by the cell plate method. It is the opinion 

 of Farr that furrowing in microsporocytes is due to conditions similar to 

 those which bring it about in animal eggs (see below), since both float 

 freely in a liquid. 



Animals. In animals there is found nothing corresponding to the 

 formation of a cell plate on the spindle fibers and its development into a 

 thick wall such as is seen in plants. As noted in the section on the 

 achromatic figure, there is often a slight differentiation at this region 

 (the "mid-body"), but it has nothing to do with cytokinesis, which is 

 brought about by simple constriction or furrowing. This process is 

 most easily followed in the segmenting egg. In small eggs, such as those 

 of worms, the daughter cells (blastomeres) round up and become more or 

 less spherical, whereas in larger eggs, such as that of the frog, a cleavage 

 furrow appears at one pole and develops through the egg without altering 

 the shape of the latter, so that the first two blastomeres have the form of 

 hemispheres. It is with animal eggs that most of the researches on the 

 mechanism of cytokinesis by furrowing have been carried out. 



Mechanism of Furrowing. Attempts to explain furrowing and the 

 separation of the daughter cells on physico-chemical grounds have been 

 rather numerous. Many years ago Biitschli (1876) advanced the view 

 that as a result of a specific activity on the part of the centrosomes cyto- 

 plasmic currents are set up which flow toward the centrosomes and 

 produce a higher surface tension at the equator of the cell, this in turn 

 bringing about furrowing and cell-division. McClendon (1910, 1913) 

 also reported an increase in surface tension at the region of furrowing. 



