66 THE SPERMATOZOON. 



tion, that a protoplasmic layer is stated to be continued round 

 the nucleus which forms the main constituent of the head. In 

 some of the elongated forms of spermatozoa, e.g. in Insecta, 

 there is no marked distinction, except in the character of the 

 protoplasm, between the head and the tail. A connecting 

 element is frequently interposed between the head and tail, 

 which appears however to be constituted of the same material as 

 the tail, and sometimes forms a thickening on the tail close 

 below the head (Amphioxus). A very remarkable modification 

 of the tail is found in many Amphibia, Reptilia and Mammalia. 

 In these types there is attached to what appears to be a normal 

 tail a delicate membrane, the outer edge of which is thickened 

 to form a kind of secondary filament. In the living spermato- 

 zoon this filament is in a state of constant movement. The 

 membrane winds spirally round the tail. 



In the majority of forms the tail of the living spermatozoon 

 exhibits sinuous cilia-like movements. In two groups the move- 

 ments are however of an amoeboid character. These groups are 

 the Nematoda and the Crustacea ; and the spermatozoa in both 

 of them frequently present very abnormal forms. In Nematoda 

 they are pear-shaped, cylindrical, spine-shaped, etc., and are 

 mainly formed of protoplasm with a highly refracting nucleus. 

 In the Crustacea the variations of form are still greater. In the 

 Malacostraca they are sometimes simply spherical (Squilla), 

 while in Astacus and a large number of Decapoda they are 

 composed of a nucleated body with stellate rays. In Paludina 

 amongst the Mollusca there are two forms of completely deve- 

 loped spermatozoa existing side by side in the same individual. 



The spermatozoa are formed by the breaking up of the male 

 germinal cells, or of cells secondarily derived from them by 

 division. The cells which directly give rise by division to the 

 spermatozoa may be called spermospores and are equivalent 

 to the ova or oospores. 



Amongst the Sponges (Halisarca, Schultze, No. 141) a 

 germinal cell, similar to that which in the female becomes an 

 ovum, repeatedly divides and eventually gives rise to a ball of 

 cells (a spermosphere or sperm-morula), each constituent 

 cell of which becomes converted into a spermatozoon, and may 

 be designated by the special term 'spermoblast.' 



