MATURATION OF GAMETES 149 



nucleus. The caudal half of the nucleus is covered by the post-nuclear cap. 

 This also appears to be a skeletal structure supporting this area of the nucleus; 

 moreover, it affords a place of attachment for the anterior centrosome and 

 the anterior end of the axial filament. 



In human and bull sperm the acrosome is a thin cap, but in some mam- 

 malian sperm it is developed more elaborately. In the guinea pig it assumes 

 the shape of an elongated, shovel-shaped affair (fig. 82), while in the mouse 

 and rat it is hatchet or lance shaped (fig. 74N). In passerine birds the acrosome 

 is a pointed, spiral structure often called the perforatorium (fig. 78). On 

 the other hand, in other birds, reptiles, and amphibia it may be a simple, 

 pointed perforatorial structure (figs. 74, 78, 79). In certain invertebrate 

 species, it is located at the caudal or lateral aspect of the nucleus (figs. 75, 78 ) . 



b) Neck. The neck is a constricted area immediately caudal to the pos- 

 terior nuclear cap and between it and the middle piece. Within it are found 

 the anterior centriole and the anterior end of the axial filament. In this par- 

 ticular region may also be found the so-called neck granule. 



c) Connecting Body or Middle Piece. This region is an important 

 portion of the sperm. One of its conspicuous structures is the central core 

 composed of the axial filament and its surrounding cytoplasmic sheath. At 

 the distal end of the middle piece, the central core is circumscribed by the 

 distal, or ring centriole. Investing the central core of the middle piece is the 

 mitochondrial sheath. The enveloping cytoplasm is thicker to some degree 

 in this area of the sperm than that surrounding the head. 



d) Flagellum. The flagellum forms the tail or swimming organ of the 

 sperm. It is composed of two general regions, an anterior principal or chief 

 piece and a posterior end piece. The greater part of the axial filament and its 

 sheath is found in the flagellum. A relatively thick layer of cytoplasm sur- 

 rounds the filament and its sheath in the chief-piece region of the flagellum, 

 but, in the caudal tip or end piece, the axial filament seems to be almost 

 devoid of enveloping cytoplasm. The end piece often is referred to as the 

 naked portion of the flagellum. 



In figure 79 is shown a diagrammatic representation of a urodele amphibian 

 sperm. Two important differences from the mammalian sperm described above 

 are to be observed, namely, the middle piece is devoid of mitochondria and 

 is composed largely of centrioles 1 and 2, and the tail has an elaborate undu- 

 lating or vibratile filament associated with the chief piece. 



3) Spermiogenesis or the Differentiation of the Spermatid into the Mor- 

 phologically Differentiated Sperm. The differentiation of the spermatid into 

 the fully metamorphosed sperm is an ingenious and striking process. It involves 

 changes in the nucleus, during which the latter as a whole contracts and in 

 some forms becomes greatly elongated into an attenuant structure. (See figs. 

 79B-F; 85L-P.) It also is concerned with profound modifications of the cyto- 

 plasm and its constituents; the latter changes transform the inconspicuous 



