208 



INTRODUCTION TO CYTOLOGY 



spermatozoid is organized. Even before the division of the body-cell 

 nucleus the enlarging blepharoplasts become vacuolate. After the 

 division is completed, each of them breaks up into a number of small 

 fragments which then coalesce and begin the formation of the spirally- 

 coiled band that bears the many cilia. This band-like blepharoplast 

 continues to elongate just within the cell membrane until it has made 

 several turns, a prolongation of the nucleus remaining in contact with its 

 growing end. The mature spermatozoid consists of an enormous 

 nucleus, which in the living condition can be seen to undergo amoeboid 

 changes of shape, a cytoplasmic sheath, and the blepharoplast with its 

 ciha. 



Fig. 125. — Microsporogenesis and development of male gametophyte in an angiosperm. 

 First row: two meiotic mitoses in microsporocyte, followed by cleavage of protoplast into 

 four microspores, three of which are shown. Second row: formation of male gametophyte 

 by one microspore, t, tube cell; g, generative cell; cT, male gametes; i, in tine; e, exine; 

 p, germ pore with plug. Semidiagrammatic. 



In the conifers, as in the cycads, the body-cell divides to form two 

 male gametes, but here they are non-motile. In some genera (Taxodium) 

 a wall separates the two nuclei after mitosis in the body-cell, but in others 

 (Pinus) these nuclei lie together in the body-cell cytoplasm. As the 

 body-cell moves down the pollen tube its outlines may remain distinct, 

 but in some cases its cytoplasm becomes intermingled with that of the 

 pollen tube. In the latter event the male gametes are simply nuclei, 

 rather than complete cells. In some genera the two male gametes are 

 differentiated in size and apparently in function, a tendency which 

 reaches its fullest expression in the Podocarpinese. 



The theory that the blepharoplasts of bryophytes and vascular plants 

 are homologous with centrosomes^^ was at one time much disputed, but 

 its validity is now generally conceded. A survey of the known cases 

 suggests that centrosomes originally functioning in the ordinary way 

 through several mitoses have tended to become restricted to one, and 

 " Belajeff (1897), Ikeno (1898 et seq.). See the review by Sharp (1912). 



