THE DEVELOPMENT OF ORGANISMS 721 



locomotor energy, and persistent vitality, it resembles a flagellate 

 Monad, whereas an ovum is comparable to a hungry Amoeba or to 

 one of the more encysted Protozoa, such as a Gregarine. 



A typical spermatozoon shows (i) a "head", mostly made up of 

 the nucleus with its chromosomes, and sometimes capped by a 

 minute "acrosome" which first attaches itself to the egg-cell in 

 fertilisation ; (2) a locomotor tail, sometimes with an axial filament, 

 a fibrillated sheath, and a delicate fringe ; and (3) between head and 

 tail, a short "middle piece", in which there is usually a "centro- 

 some", regarded by many as important in the cleavage that follows 

 fertilisation. The spermatozoa of threadworms and many crusta- 



FiG. 109. 



Diagram of an Animal Spermatozoon. A, the firm peak or acrosome; H, the 

 head; Cy, the cytoplasm of the head; CHR, the nuclear chromosomes; 

 MP, the middle piece, including the centrosome (C); AF, the axial 

 filament of the locomotor tail (T). 



ceans are sluggish and inclined to be amoeboid. In the threadworms 

 there is internal fertilisation, and the sluggish sperms meet the ova 

 towards the lower end of the female genital duct, being apparently 

 wafted inwards by motile tags on the internal wall. In many of the 

 crustaceans there is internal fertilisation; in others, like the cray- 

 fish, the male may deposit the sperms on the liberated eggs which are 

 attached to some of the appendages of the female. It has often been 

 pointed out that the fixed barnacles have mobile spermatozoa, and 

 it is plain that the occurrence of sluggish sperms would be a severe 

 handicap to sedentary types. But as a matter of fact there are other 

 crustaceans besides barnacles that have mobile sperms, such as 

 many Ostracods and the water-flea Polyphemus. 



Sometimes, as among Crustaceans, there are adaptations which 

 secure fertilisation when the sperms are non-mobile; thus many 



VOL. II A 



