228 INTRODUCTION TO CYTOLOGY 



tion (trichogyne) of the female sex organ (carpogonium). Solution of the 

 intervening walls allows the nucleus of the spermatium to pass into the 

 trichogyne and down to the female nucleus in the base of the carpogonium 

 (Fig. 139, A). The male nucleus, when it reaches the female nucleus, 

 appears as a group of 20 chromosomes. In this condition it enters the 

 female nucleus while the latter is yet in the reticular state. Soon the 

 female reticulum condenses into 20 chromosomes, which arrange them- 

 selves with the 20 paternal chromosomes in the spindle as the fusion 

 nucleus divides (Yamanouchi, 1906). The spermatium of the Floridese is 

 said to carry no plastids. 



A special condition is found in the Conjugatse. Here certain vegeta- 

 tive cells, after little morphological alteration, fuse in pairs to produce 

 resting zygospores. In Sjnrogyra, for example, the entire protoplast 

 passes through a conjugating tube to unite with a similar cell, commonly 

 in another filament. The two nuclei fuse, but the plastids remain distinct 

 and those introduced by the migrating cell are observed in some species to 

 degenerate in the zygospore (Fig. 162). In some species the gametes 

 meet and form the zygospore in the conjugating tube. The movement 

 of these gametes is accomplished through the remarkable action of 

 contractile vacuoles, which withdraw water from the central sap vacuole 

 and discharge it between the protoplast and the wall.^ 



In the diatoms^ there are two general modes of sexual behavior, with 

 minor variations in the different species. With rare exceptions these 

 modes characterize the two main groups, Pennatae and Centricae. In 

 Rhopalodia gibba, a representative of the former group, two individuals 

 meet and form a common mass of jelly between them. In each individual 

 the nucleus undergoes two successive divisions, the protoplast then divid- 

 ing into two portions, each with a large and a small nucleus. These two 

 portions now fuse with the two produced by the other individual, forming 

 two zygotes which develop into large auxospores. In each case the large 

 nuclei fuse and the small ones degenerate (Klebahn). In some diatoms, 

 for example, certain forms of Cocconeis, the two uniting individuals show 

 distinct differences in size and sexual behavior (Geitler, 1927afe). In the 

 Centricae the auxospores are formed asexually, the sexual act consisting in 

 the union of small biciliate gametes. In Coscinodiscus these are of 

 two types: small colorless ones and larger ones with chromatophores 

 (Pavillard).^ 



'Lloyd (1925, 1928a). Conard (19Slh) claims that the cytoplasmic portions of 

 the gametes remain distinct and are separated by the first wall at germination. 



4 Klebahn (1896), Lauterborn (1896), Karsten (1897, 1900, 1904, 1912, 1924), 

 O. Mliller (1906), Bergon (1907), Pavillard (1913), Geitler (1927a6c, 1928a6c, 19296), 

 von Cholnoky (1927, 1928, 1929), P. Schmidt (1923, 1927a6). See Pavillard (1910) 

 and Oltmanns (1922). 



^ These small cells are termed "microspores" in the literature. The rarely 

 observed "microspores" in Pennatae seem to be asexual. 



