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INTRODUCTION TO CYTOLOGY 



These spermatozoids are very much alike in structure and behavior in 

 the two groups, and are unusually large, being easily visible to the naked 

 eye. The body is made up of a large nucleus surrounded by a thin 

 cytoplasmic layer in which is imbedded a long, spirally coiled blepharo- 

 plast bearing many cilia (Fig. 114, C). The behavior of the spermatozoid 

 in fertilization has been studied in Ginkgo by Hirase (1895, 1918) and 

 Ikeno (1901); in Cycas revoluta by Ikeno (1898); in Zamia floridana by 

 Webber (1901); and in Dioon edule, Ceratozamia mexicana, and Stangeria 



paradoxa by Chamberlain (1910, 1912, 1916). 

 In all cases the entire spermatozoid penetrates 

 into the egg cytoplasm, where the nucleus frees 

 itself from the cytoplasmic sheath with its 

 blepharoplast and cilia and advances alone to 

 the egg nucleus, with which it fuses (Fig. 119). 

 The behavior of the chromatin during the fusion 

 is not well known in either Ginkgo or the cycads. 

 In the Coniferales and Gnetales the male 

 cells have no motile apparatus. Each consists 

 of a nucleus surrounded by a more or less 

 sharply delimited mass of cytoplasm. In most 

 cases this cytoplasm remains intact until after 

 the male cell has entered the egg, but in other 

 forms, such as Pinus, it mingles with the cyto- 

 plasm 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 dis- 

 charged into the egg. All but the functioning 

 male nucleus usually degenerate at once, but in 

 some cases they have been observed to undergo 

 division. 

 When a complete male cell enters the egg the cytoplasm of the former 

 shows two general modes of behavior. In some species it may be left 

 behind in the peripheral region of the egg as the male nucleus frees itself 

 and advances alone to the female nucleus. This type of behavior has 

 been reported in Pinus (Ferguson 1901, 1904), Thuja (Land 1902), 

 Juniperus (Noren 1904), Cryptomeria (Lawson 1904), and Libocedrus 

 (Lawson 1907). In Sequoia (Lawson 1904) the male nuclei escape from 

 their cytoplasm before their discharge from the pollen tube, and enter 

 the egg alone. In a second group of species the male cytoplasm remains 

 intact and invests the fusing sexual nuclei, being clearly distinguishable 

 from the cytoplasm of the egg. The pollen tube cytoplasm often plays 



Fig. 119. — Fertilization 

 in Zamia. Male nucleus 

 uniting with egg nucleus at 

 center; cytoplasmic^sheath 

 with spiral blepharoplast 

 above. Another sperm 

 outside egg. X 25. (After 

 Webber, 1901.) 



