THE SPERMATOZOON. 53 



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 spermatozoon 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 movements are 

 however of an amoeboid character. These groups are the Nema- 

 toda 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 developed 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.' 



In most Hydrozoa the subepithelial epiblastic cells become con- 

 verted into germinal cells (spermospores), and then break up to 

 form spermoblasts, each of which becomes a spermatozoon. 



In most higher Metazoa the spermospores usually form the 

 epithelium of an ampulla or tube, though more rarely (many 

 Cha^topoda, Gephyrea, etc.) they may be derived from cells lining 

 the body-cavity, as in the case of ova. The spermatozoa are 

 formed either by the direct division of the spermospores into a 

 number of cells, spermoblasts, each of which grows into a sperma- 

 tozoon ; or by the nucleus of the spermospore becoming subdivided 

 within the cell body, the latter differentiating itself into the tails 



