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BOTANY 



The vacuoles now fuse to form larger vacuoles, which practically 

 separate the protoplasm into four parts or masses. Furrows are 

 now formed at the surface and proceed to grow inwards and meet 

 the vacuoles, and complete cleavage of the protoplast is effected. 

 At the heterotypic division callose is secreted in large amounts 

 by the protoplast, which is only first seen at the corners of the 

 pollen-mother cells as synizesis is initiated. With further develop- 



FiG. 75. — Cytokinesis in the microsporocyte of Melitotiis. (1) Prophase of 

 first meiotic division (callose uniformly stippled) being secreted from 

 protoplast. (2) Telophase of homotypic division. (3) Special wall (black) 

 and fissures appearing. (4) and (5) Vacuoles forming between nuclei. 

 (6) Special wall extending in wards. (7) Special wall extensions have 

 met at centre. (8) Thickened special wall complete. (After Castetter, 

 from Sharp, Introduction to Cytology.) 



inent of the nucleus the protoplast becomes entirely surrounded 

 by this homogeneous substance, until in the pachynema stage it 

 becomes quite massive. At the end of the homotypic division, 

 and as the cleavage furrows near completion, this special callose 

 wall advances centripetally with the furrows which soon cut into 

 the vacuoles. Finally, partitions, which are the continuation of 

 the incoming callose wall, are formed between the young micro- 

 spores. These partitions are formed by the deposition of a callose 

 secretion of the protoplasts on the surface of the wall (Fig. 75). 



