92 



BOTANY 



PART I 



3. TuK Cell in the Process of Fertilisation 



III the organic world when a definite degree of phylogenetic de- 

 velopment has been attained we find the development of a sexual 

 differentiation in the organisms. Sexual cells are formed which are in- 

 capable of independent development and are 

 only able to continue their 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 distinguished as egg and sperm ATOZOID. 



The spermatozoid is the male, the 



egg 



the 



Fin. 98 — A, An asexual swarm-spore 

 oi Ulothrix zonata ; /J, 1, a gamete ; 

 2 and 3 conjugating gametes ; 4, 

 zygote, formed by the fusion of 

 two gametes. (x-IOO.) 



Fio. 97. — Successive stages of the delimitation of a spore in the 

 ascus of Erysiphe communis. A, Before delimitation has begun ; 

 the librillar plasma (kp) radiates into the cytoplasm around ; 

 in £ the fibrillar plasma has commenced to grow round the 

 mass of cytoplasm ; in C this process is coniijlete, but the 

 limiting layer produced by the fusion of radiations of tlie 

 fibrillar plasma is still connected with the polar side of tlie 

 niicleus ; in I) tliis kinoplasmatic connection lietwcen nucleus 

 and limiting layer has disappeared ; s, nuclear network ; n, 

 nucleolus. (After Harper, x 1500.) 



Fio. 99 — A, Spermatozoid of C'liara 

 fragilis ; Ji, Spermatozoid of *he 

 Fern, Onocleastrnthiopteris. (Aiter 

 Shaw.) The nucleus (/.) is more 

 deeply shaded ; c, the cytoplasmic 

 portion ; cl, cilia which in J) arise 

 from the denser edge of the spiral 

 band ; b, vesicle. (A x 540, 

 B X 850.) 



female sexual cell . The gametes may be motile (Fig. 98 B) or non-motile. 

 The motile gametes frequently resemble the swarm-spores (Fig. 98 y1) 

 generated by the same parent for the purpose of asexual reproduction. 

 As a rule, however, they are smaller than the swarni-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 



