CHAPTER II. 

 THE EARLY DEVELOPMENT OF MAMMALS. 



The Spermatozoon. 



The spermatozoa of mammals are filaments consisting of a short thick end 

 called the head, and a very long, delicate thread called the tail. They are of 

 minute size as compared with the ovum. The head varies greatly 

 'in shape according to the species. It contains chromatin, hence 

 it stains darkly with those histological dyes which color nuclei. 

 The tail consists of three parts: first, the middle piece, which is 

 next the head, is short and the thickest of the three parts, contains 

 an axial thread, and probably always has a very fine spiral thread 

 running round it; second, the main piece, which is the longest 

 part of the tail; and, third, the end piece, which is not more 

 than a line, even as seen with very high microscopic powers. 



The Human Spermatozoon. The human spermatozoon is 0.055 

 mm. long the head being 0.005 mm., the tail 0.050, and the 

 middle piece 0.009. It is shown in two views in figure 2. The 

 head is flattened and pointed. Seen from the flat side, it appears 

 oval (Fig. 2, A) with the front end generally tapering a little, but 

 never pointed. The anterior half or two thirds has a brighter 

 and more transparent part. Seen on edge (Fig. 2, B), the 

 head has a pointed form with a posterior, thicker, round dark 

 part. By adjustment of the focus it can be ascertained : that the 

 sides near the point are depressed. Some writers maintain that 

 there is a special tip projecting from the head as a cylinder 

 thread, with a hook at its end. The middle piece, mi, is directly 

 united with the head by a transverse joint. It is cylindrical and 

 about as long as, or a little longer than, the head. Its surface is 

 often granular or rough, and there cling to it a few shreds of 

 protoplasm. It has a spiral thread, which is easily overlooked 

 on account of its extreme fineness. The main piece, m, of the 

 tail is about half as thick as the middle piece. It gradually 

 tapers and ends abruptly at the beginning of the still finer and very short end 

 piece, e. The tail probably contains an axial thread, as has been observed in other 

 3 33 



FIG. 2. HUMAN 

 SPERMATOZOA. 



A, Complete sperma- 

 tozoon. B, Head 

 seen from the side. 

 C, Extremity of 

 the tail, h, Head. 

 mi, Middle piece. 

 m, Main piece, e, 

 End piece. A 11 

 highly magnified . 

 (After Retzius.) 



