GAMETOGENESIS AND SPOROGENESIS 



205 



in each cell of the penultimate generation; there it divides to two, which 

 separate and occupy the poles of the achromatic figure in the manner of 

 animal centrosomes. At the close of mitosis the blepharoplast in each 

 spermatid fragments into a number of pieces which later join to form a 

 continuous chromatic thread. The spirally coiled body of the mature 

 spermatozoid in pteridophytes consists of the elongated nucleus, a 

 cytoplasmic band lying along the anterior portion of the nucleus and 

 containing the chromatic thread at its margin, a posterior globule of 

 cytoplasm containing starch and other inclusions, and numerous cilia. 

 Students of spermatozoid development have described the cilia as 

 growing out from the chromatic thread ("blepharoplast"), though 



Fig. 122. — Spermatogenesis in Equisetum arvense, showing the behavior of the blepharo- 

 plast (centrosome) in the last spermatogenous mitosis and in the transformation of the 

 spermatid into the spermatozoid. {After Sharp, 1912.) 



recent observers of mature spermatozoids report that they appear to 

 be attached to basal bodies in the cytoplasmic band whose margin is 

 formed by the thread ("border brim") (Dracinschi, Yuasa). 



In Marsilia each of the two primary spermatogenous cells undergoes a 

 series of four divisions which result in 16 spermatids. No distinct centro- 

 somes appear during the first mitosis. During the anaphase of the second 

 mitosis centrosomes develop at the spindle poles. In the telophase they 

 divide but usually then disappear in the cytoplasm. In the third mitosis 

 centrosomes again appear at the spindle poles during the anaphase, and in 

 the telophase they divide. The daughter centrosomes so formed then 

 move apart and occupy the poles of the spindle through the fourth, or 

 final, mitosis. They are at all times accompanied by distinct cytoplasmic 

 radiations, the achromatic figure being strikingly like those in animals and 

 certain thallophytes. Before the last mitosis is completed, the centro- 

 some (blepharoplast) becomes vacuolate, and in the spermatid it breaks 



