i INTRODUCTION 7 



which this change is effected seems to be fundamentally the same in 

 most animals which have been examined. One of the best and most 

 recent accounts of it is that given by Duesberg (1909), who worked 

 on the development of the spermatozoa of the rat. According to this 

 author the spermatid is a small polygonal cell containing a large 

 resting nucleus, at one side of which is the centrosome which 

 functioned in the last maturation division. This centrosome has 

 already divided into two minute centrioles lying one above 

 the other (Fig. 2, A, c 1 , c). Besides the centrioles there is a 

 peculiar body embedded in the cytoplasm termed the idiosome, 

 apparently derived from the sphere or modified cytoplasm which sur- 

 rounded the ceutrosome, although it is now widely separated from the 

 centrioles. The distal or outer centriole begins to give rise to a thin 

 filamentous fiagellum which is the rudiment of the tail of the 

 spermatozoon, whilst the idiosome travels to the opposite side of the 

 nucleus from that on which the centrioles lie, and becomes applied 

 to the nuclear membrane and forms a cap-like structure known 

 as the acrosome. The nucleus sends out a prolongation which 

 reaches the lower or proximal centriole, and this becomes applied to 

 the nuclear membrane and forms a plate-like thickening on it. The 

 nucleus carrying the acrosome then begins to emerge from the 

 cytoplasm on the opposite side of the cell to that on which the 

 ceutrioles are situated. Both centrioles are dragged after the 

 nucleus. Round the upper or immersed half of the nucleus the 

 cytoplasm is differentiated so as to form a funnel of clear substance 

 called the " ruffle " (manchette) (Fig. 2, D, m). The nuclear sap is 

 then apparently expelled and the nucleus converted into an almost 

 uniform mass of staining matter. Its form changes no longer 

 spherical, it becomes sickle-shaped (Fig. 2, E). The distal centriole 

 divides into two daughter centrioles, and to the lower and proximal 

 of these, the tail filament, now greatly grown in length, is attached, 

 whilst the upper or distal one forms a ring surrounding the filament. 

 The nucleus now emerges completely from the cytoplasm. It forms 

 the head of the spermatozoon ; the ruffle disappears, but the acre- 

 some is still distinguishable as a thickening on the convex side of 

 the sickle. The cytoplasm now forms a thick mantle surrounding 

 the lower part of the filament (Fig. 2, G). The ring-shaped centriole 

 travels away from the nucleus along the filament till it reaches 

 a definite position. The cytoplasm shrinks more and more and is 

 eventually completely cast off. The ring-shaped centriole and the 

 piece of the filament between it and the nucleus forms the middle- 

 piece of the spermatozoon. When these changes are complete the 

 spermatid is transformed into a spermatozoon, begins to exhibit 

 active movement, and is capable of fertilizing a ripe egg. 



We must now turn our attention to the process of maturation of 

 the egg. We left the egg in the stage when the nucleus was 

 inordinately swollen with cell sap, and when there was a large 

 nucleolus. All the changes exhibited by the ripening sperinatocyte 



