2l8 FERTILIZATION OF THE OVUM 



more or less widely at the time of union, the sperm-nucleus being 

 smaller, more compact, and deeply staining (Figs. 105, 108), as is the 

 case in such forms of fertilization as the echinoderm-egg. In the 

 case of angiosperms all earlier observers, including Strasburger ('78, 

 '84), Guignard ('91, i), and Mottier ('97, i), found the conjugating 

 nuclei to be closely similar at the time of union. The recent obser- 

 vations of Guignard ('99) and Nawaschin ('99) show, however, that 

 even here the sperm-nucleus is smaller, more compact, and of differ- 

 ent form (spindle-shaped) from the egg-nucleus (Fig. 107). 



The ovum or oosphere of the flowering plant is a large, rounded 

 cell containing a large nucleus and numerous minute colourless 

 plastids from which arise, by division, the plastids of the embryo 

 (chromatophores, amyloplasts). In the angiosperms the ovum forms 

 one of the eight cells constituting the embryo-sac which morphologi- 

 cally represents the female prothallium or sexual generation of the 

 pteridophyte and is itself embedded in the ovule within the ovary. 1 

 The male germ-cells are represented in the cycads by two ciliated 

 spermatozoids (p. 175), in the angiosperms by two spindle-shaped 

 "generative nuclei" which are suspected by Guignard and Nawaschin 

 to be motile bodies, though no cilia were seen. These lie near the 

 tip of the pollen-tube (Fig. 107), which is developed as an outgrowth 

 from the pollen-grain and represents a rudimentary male prothallium 

 or sexual generation. 2 



The formation of the pollen-tube, and its growth down through 

 the tissue of the pistil to the ovule, was observed by Amici ('23), 

 Brongniart ('26), and Robert Brown ('31); and in 1833-34 Corda was 

 able to follow its tip through the micropyle into the ovule. 3 Stras- 

 burger first demonstrated the fact that the generative nucleus, carried 

 at the tip of the pollen-tube, enters the ovum and unites with the egg- 

 nucleus, and the facts have been since carefully studied by himself, 

 by Guignard, Mottier, Webber, Ikeno, Hirase, and a number of others. 

 In the cycads, according to the last-named two observers, a single 

 spermatozoid enters the egg, its nucleus soon fusing with that of the 



1 The eight cells are at first arranged in an upper and a lower " tetrad " of four cells each, 

 the former including the ovum, two synergidae, and an " upper polar cell," the latter a 

 "lower polar cell" and three antipodal cells (Figs. 106, 107); cf. p. 263. 



2 Cf. p. 264. 



3 It is interesting to note that the botanists of the eighteenth century engaged in the same 

 fantastic controversy regarding the origin of the embryo as that of the zoologists of the 

 time. Moreland (1703), followed by Etienne Francois Geoff roy, Needham, and others, 

 placed himself on the side of Leeuwenhoek and the spermatists, maintaining that the pollen 

 supplied the embryo which entered the ovule through the micropyle (the latter had been 

 described by Grew in 1672) ; and even Schleiden adopted a similar view. On the other 

 hand, Adanson (1763) and others maintained that the ovule contained the germ which was 

 excited to development by an aura or vapour emanating from the pollen and entering through 

 the tracheae of the pistil. 



