SECT, i MORPHOLOGY 91 



Ephedra, for example, and also in the formation of the spores of the Ascomycetes. 

 A single nucleus is present to begin with in each ascus of the Ascomycetes. By 

 successive divisions eight nuclei lying free in the cytoplasm are derived from this. 

 A definite portion of cytoplasm around each of these nuclei becomes limited by a 

 peripheral layer which then forms a cell wall. Thus eight separate spores arise (cf. 

 Fig. 327). As the researches of Harper ( 86 ) have shown, the formation of the peri- 

 pheral layer proceeds from a centrosome-like mass of kinoplasm which formed a pole 

 of the spindle in the preceding nuclear division. The nucleus is drawn out towards 

 this mass of kinoplasm. From the latter kinoplasmic radiations proceed which 

 surround the spore as it becomes delimited, and finally fuse to form its peripheral 

 layer (Fig. 96). 



Cell - Budding. This is simply a special variety of ordinary cell division, in 

 which the cell is not divided in the middle, but, instead, pushes out a protuberance 

 which, by constriction, becomes separated from the mother cell. This mode of cell 

 multiplication is characteristic of the Yeast plant (Fig. 2, p. 11) ; and the spores, 

 known .as conidia, which are produced by numerous Fungi, have a similar origin 

 (Fig. 353). 



Conjugation ( 87 ). A sexual cell is, with few exceptions, only able to continue 

 its development after fusion with another sexual cell. The two cells so uniting 

 are either alike, and in that case are called GAMETES, or unlike, and are then dis- 

 tinguished as EGG and SPERMATOZOID. The spermatozoid is the male, the egg the 

 female sexual cell. The gametes may be motile (Fig. 97 B) or non-motile. The 

 motile gametes frequently resemble the swarm-spores (Fig. 97 A) generated by 

 the same parent for the purpose of asexual reproduction. As a rule, however, they 

 are smaller than the swarm-spores, and have usually only half as many cilia. In 

 the more highly specialised sexual cells the egg usually retains the structure of an 

 embryonic cell, but the spermatozoid undergoes profound modifications. A 

 cytoplasmic cell body, a nucleus, and the rudiments of chromatophores are 

 always present in the egg. The spermatozoid (Fig. 98), on the other hand, 

 becomes transformed, in the more extreme cases, into a spirally twisted body, 

 provided with cilia, and exhibiting an apparently homogeneous structure. Only 

 a knowledge of the history of its development, and the greatest care in hardening 

 and staining, have rendered it possible to recognise the homology of the structure 

 of such a spermatozoid with that of an embryonic cell. It has been shown that 

 the hinder part of its spiral body corresponds to the cell nucleus (k), the anterior, 

 together with the cilia, to the cytoplasm, especially the kinoplasm (c), and the 

 vesicle (I), at the other extremity, to the sap cavity of a cell ( 8tJ ). 



Motile male cells provided with cilia occur only in the Cryptogams and, as has 

 been recently demonstrated ( 89 ), in some Gymnosperms (Cycadaceae, Ginkgo). In 

 the Cryptogams the spermatozoids are set free from the sexual organs and require 

 water for their dispersal. They reach the egg-cell, which usually remains in its 

 place of origin, by swimming. In the Gymnosperms, which form motile spermato- 

 zoids, the latter are brought near to the ovum by means of the pollen tube developed 

 from the pollen grain. In a similar way the non-motile male cells of the other 

 Gymnosperms and the Angiosperms are conducted to the egg through the pollen 

 tube (Fig. 99). In the union of the two sexual cells in the act of fertilisation, the 

 egg nucleus (ek) and the sperm nucleus (sk) fuse and form the nucleus of the fertilised 

 egg-cell. The cytoplasm of the male cell also commingles with that of the female 

 cell, but the chromatophores of the embryo are derived from the egg-cell alone. 

 When the spermatozoid, as in animals and in Thallophytes, is provided with a 

 centrosome, this does not fuse with the centrosome belonging to the ovum. The 



