MALE REPRODUCTIVE CELLS 429 



This flagellum, or tail, as it is known, consists of a strong axial fila- 

 ment which shows a fibrillar structure like that of smooth muscle. The 

 cytoplasm covers it as a sheath, except for a part of its distal end, which 

 has been called the end-piece. 



Developed in or by the cytoplasmic sheath is a particular struc- 

 ture intended to give the tail a proper resistance to the fluid through 

 which it swims. This is the fin, and may assume a number of peculiar 

 forms. It is usually a filament or ribbon of considerable length, pro- 

 jecting as a flange from the cytoplasmic sheath around which it is spi- 

 rally wound. 



Many spermatozoa are of a widely different type of structure. The 

 simplest is an amoeboid form found in some Crustacea. Other Crustacea 

 have very peculiar kinds, all of which seem to be constructed on a radial 

 plan, with from three to twelve or more processes that cannot be called 

 cilia or flagella on account of their structure. They are permanent 

 cytoplasmic processes. 



The changes through which the spermatid develops into the sperma- 

 tozoon are well demonstrated in the following pages. We shall outline 

 them in a few words at this point. 



The spermatid is, at first, a very ordinary-looking cell, rather smaller 

 in size than the average and with no suggestion about it of the sperma- 

 tozoon form. Its first step in development is the appearance on one edge 

 of a dot, the future end-knob, from which a tiny filament grows out dis- 

 tally. As the filament, which is the future tail, and the granule giow in 

 size, they push in toward the nucleus, which the end-knob almost touches. 

 A ring forms around the proximal part of the tail which now becomes 

 the middle-piece, and the nucleus becomes compact and apparently loses 

 its reticular chromatin. It moves into an eccentric position in the cyto- 

 plasm, and this region later becomes the head of the spermatozoon. 

 The nucleus may assume a number of forms before finally becoming 

 the head of the adult spermatozoon. The ring usually elongates and 

 becomes spirally arranged about the tail. The axial filament has 

 been shown to originate from a centrosome ray. Sometimes a sec- 

 ondary spermatocyte makes a weak attempt to form a tail in this 

 way. 



During this elaborate development of highly differentiated motor 

 structures, the cell shows a varying ability in the power to nourish itself. 

 Many spermatids apparently find no difficulty in securing food from the 

 surrounding fluids, while others resort to the same methods that most 

 developing ova do, and attach themselves to a nurse cell. In this case, 

 however, one nurse cell becomes the feeder of many spermatids which, 

 on account of their small size, need but little food. The connection is 

 usually not established until the spermatid stage is somewhat advanced. 



