THE CENTROSOME AND THE BLEPHAROPLAST 



91 



plast in each spermatid fragments into a number of pieces; these later 

 join to form a continuous beaded thread from which the cilia grow out. 

 In Equisetum the elongating nucleus and blepharoplast do not become 

 closely joined, but are held together only by the rather abundant cyto- 

 plasm. The spermatozoid is multiciliate like that of all other pterido- 

 phytes with the exception of Lycopodium, Phylloglossum, and Selaginella: 

 in these three genera the spermatozoids are biciliate like those of the 

 bryophytes. 



The most careful work on the blepharoplast of homosporous ferns is 

 that of Yamanouchi (1908) on Nephrodium (Fig. 31). In this form no 

 centrosomes are found. The two blepharoplasts, 

 which arise de novo in the cytoplasm of the sperm- ^- : 



atid mother-cell, take no active part in nuclear 

 division, merely lying near the poles of the spindle. 

 In the spermatid the blepharoplast elongates spirally 

 in close union with the nucleus to form the body of 

 the spermatozoid. In Adiantum and Aspidium Miss 

 R. F. Allen (1911) and Thorn (1899) see the blepharo- 

 plast first in the spermatid. 



One of the most interesting blepharoplasts is that 

 of Marsilia (Fig. 32), first described by Shaw (1898). 

 According to Shaw a small granule, or "blepharo- 

 plastoid," appears near each daughter nucleus of the 

 mitosis which differentiates the grandmother-cell of 

 the spermatid (the second of the four spermatogenous 

 mitoses). During the next (third) division the 

 blepharoplastoid divides but soon disappears, and a 

 blepharoplast appears near each spindle pole. In the 

 next cell generation (spermatid mother-cell) the 

 blepharoplast divides into two which are situated at 

 the spindle poles during the final mitosis. In the 

 spermatid the blepharoplast gives rise to several 

 granules by a sort of fragmentation; these together 

 form a thread which elongates spirally in close union 

 with the nucleus and bears many cilia. The spermatozoid is of the usual 

 fern type, with several coils and a cytoplasmic vesicle. Shaw saw in the 

 foregoing facts no ground for the homology of the blepharoplast and the 

 centrosome. Belajeff (1899) found that centrosomes occur at the spindle 

 poles during all, excepting possibly the first, of the four divisions which 

 result in the 16 spermatids. He reported that after each mitosis the 

 centrosome divides into two which occupy the spindle poles during the 

 succeeding mitosis, and in the spermatids perform the usual functions of 

 blepharoplasts. Belajeff regarded this as a strong confirmation of his 

 theory that the blepharoplast and centrosome are homologous organs. 



FIG. 31 . Two 

 stages in the sperma- 

 togenesis of Nephro- 

 dium. 



A, blepharoplasts 

 near poles of spindle 

 in last spermatogen- 

 ous mitosis. B, elon- 

 gation of blepharo- 

 plast near nucleus. 

 Nebenkern at left. 

 {After Yamanouchi, 

 1908.) 



