234 



INTRODUCTION TO CYTOLOGY 



presumably the blepharoplast and most of the cytoplasm) are left in the 

 cytoplasm as the compact and coiled male nucleus enters the egg nucleus 

 (Fig. 139, C). The male nucleus then gradually becomes reticulate, and 

 eventually the materials contributed by the two gametes become 

 indistinguishable. 



The behavior of the plastids during gametogenesis and syngamy in 

 ferns is not well known. Plastids may frequently be seen in the develop- 

 ing egg, and presumably they play a role in 

 spermatogenesis somewhat like that in bryophytes 

 (p. 203). At present it seems probable that the 

 plastids of the sporophyte are derived chiefly if 

 not wholly from the female gamete. 



Gymnosperms. — In the cycads and Ginkgo the 

 pollen tube discharges two motile spermatozoids 

 near the archegonium, into which one of them 

 passes by squeezing between the neck cells. The 

 nucleus of the spermatozoid then becomes free 

 from its cytoplasmic sheath, blepharoplast, and 

 cilia and advances alone to the egg nucleus, with 

 which it fuses^^ (Fig- 142). In Ginkgo the cyto- 

 plasm and motor apparatus are left in a "receptive 

 spot" formed by vacuoles in the upper part of the 

 egg. In Bowenia the paternal and maternal 

 chromosomes can be distinguished in the first few 

 mitoses preceding wall formation in the embryo. 

 In the Coniferales and Gnetales the male cells 

 have no motile apparatus. Each consists of a 

 nucleus surrounded by a more or less sharply 

 matozoid above; another dehmited mass of cytoplasm. In most cases this 

 spermatozoid at surface of cytoplasm remains intact until after the male 

 egg. {After Wehher, 1901.) ^^^^ ^^^ entered the egg, but in other forms it 



mingles with the cytoplasm of the pollen tube, so that only male nuclei, 

 rather than completely organized male cells, are delivered to the egg. 

 All the nuclei present in the pollen tube — stalk nucleus, tube nucleus, 

 the two male nuclei, and, in certain species, free prothallial nuclei — may 

 be discharged into the egg. All but the functioning male nucleus usually 

 degenerate at once, but in some cases they have been observed to undergo 

 division. 



Male cells of conifers show two general modes of behavior after their 

 entrance into the egg. In some instances the male's cytoplasm is left 

 in the peripheral region of the egg while the nucleus advances alone to 



i«Hirase (1895, 1898, 1918), Ikeno (1901), Shimamura (1928) and Herzfeld 

 (1927) on Ginkgo; Ikeno (1898) and Kuwada (1925o, 19266) on Cycas; Chamberlain 

 (1910, 1912, 1916) on Dioon, Ceratozamia, and Stangeria; Lawson (1926) on Bowenia. 



Fig. 142. — Syngamy in 

 Zamia. Male and female 

 nuclei uniting at center; 

 cytoplasmic sheath and spi 

 ral blepharoplast of sper 



