THE SPERMATOZOON 



135 



Its precise study is therefore difficult, and it is not surprising that our 

 knowledge of its structure and origin is still far from complete. 



Apical body or acrosome. 



Nucleus. 



End-knob. 



' Middle-piece. 



Envelope of the tail. 



. .\xial filament. 



I. Flagellate Spermatozoa 



In its more usual form the animal spermatozoon resembles a 

 minute, elongated tadpole, which swims very actively about by the 

 vibrations of a long, slender tail morj^ho- 

 logicall)' comparable with a single cilium 

 or flagellum. Such a spermatozoon con- 

 sists typically of four parts, as shown in 

 Fig. 65: — 



1. The nucleus, which forms the main 

 portion of the "head," and consists of a 

 very dense and usually homogeneous mass 

 of chromatin staining with great intensity 

 with the so-called "nuclear dyes" {e.g. 

 haematoxylin or the basic tar-colours such 

 as methyl-green). It is surrounded by a 

 very thin cytoplasmic envelope. 



2. An apical body, or acrosome, lying at 

 the front end of the head, sometimes very 

 minute, sometimes almost as large as the 

 nucleus, and in some cases terminating in 

 a sharp spur by means of which the 

 spermatozoon bores its way into the ovum. 



3. The middle-piece, or connecting 

 piece, a larger cytoplasmic body lying 

 behind the head and giving attachment to 

 the tail, from which it is not always dis- 

 tinctly marked off. This body shows the 

 same staining-reactions as the acrosome, 

 having an especial affinity for " plasma- 

 stains " (acid fuchsin, etc.). At its front 

 end it is in some forms (mammals) sepa- 

 rated from the nucleus by a short clear 

 region, the neck. Like the acrosome, the 

 middle-piece is in some cases derived from an " archoplasmic " mass, 

 representing an attraction-sphere {Lumbricus) or a portion of the 

 Nebenkern (insects), and it contains, or according to some authors 

 actually arises from, the centrosome (salamander, mammals, insects, 

 etc.). 



4. The tail, or flagellum, in part, at least, a cytoplasmic product 

 developed in connection with the centrosome and " archoplasm " 



End-piece. 



Fig. 65. — Diagram of the 

 flagellate spermatozoon. 



