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 run- 

 ning 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 rnm., the tail 0.050, 

 and the middle piece 0.009. It is shown in two views in 

 Fig. I. The head is flattened and pointed. Seen from the 

 flat*side, it appears oval (Fig. i, A) with the front end gener- 

 ally tapering a little, but never pointed. The anterior half 

 or two-thirds has a brighter and more transparent part. 

 Seen on edge (Fig. i, 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 

 c. Extremity of the depressed, somcwhat like those of human red blood-corpuscles. 

 ' *"^' Some writers maintain that there is a special tip projecting 



Fig. I. — Human 

 Spermatozoa. 



A, Complete sperma- 

 tozoon. B, Head 

 seen from the side. 



Middle piece, w, 

 Main piece, e. End 

 piece. All highly 

 magnified. — {^Afier 

 Retzius. ) 



from the head as a cylinder thread, with a hook at its end. 

 The middle piece, wt, 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 



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