168 



INTRODUCTION TO CYTOLOGY 



By some observers the special wall has been thought to arise through a 

 swelling of the secondary layers of the sporocyte wall, but it now seems 

 evident that it is secreted directly by the protoplast (Gates, Castetter, 

 Woycicki), which may undergo a distinct contraction at this time. 

 Eventually the sporocyte wall and the material separating the four 

 microspores disappear, leaving the spores free from one another. 



Cytokinesis in the microsporocyte of Melilotus alba is of interest in 

 that vacuoles appear to play a conspicuous part in the process (Fig. 96). 

 After the second mitosis small vacuoles develop in the regions between 

 the four nuclei and fuse to form larger ones which nearly separate the 

 protoplasm into four masses. Furrows originating at the surface then 



Fig. 96. — Cytokinesis in the microsporocyte of Melilotus. 1, prophase of first meiotic 

 mitosis; callose (uniformly stippled) being secreted by the protoplast. 2, telophase of 

 second mitosis. 3, special wall (black) and furrows appearing. 4, 5, vacuoles forming 

 between nuclei. 6, special wall extending inward. 7, special wall extensions have met at 

 center. 8, thickened special wall complete. {After Castetter, 1925.) 



grow inward, meet the vacuoles, and complete the cleavage of the proto- 

 plast (Castetter). A similar process occurs in Gentiana (Woycicki). 



In Magnolia, Farr finds a case in which bilateral quartets are formed 

 by furrowing, rather than by cell-plates as might be expected. Although 

 a transitory cell-plate may be differentiated, it plays no part in cyto- 

 kinesis. 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 and forms a partition through the equator of the sporocyte. 

 At the same time additional furrows subdivide the two hemispheres, thus 

 delimiting the four microspores. 



Furrowing in Animals. — Cytokinesis, which in animals is most 

 commonly accomplished by furrowing, has been studied with special 

 care in segmenting eggs. In the case of small eggs, such as those of 



