STRUCTUKE OF SPERMATOZOON 5 



daughter- V's next get free from one another and pass to the apices of the spindle 

 where they gather in groups round the poles (fig. 9). They then merge together 

 again to form the reticulum of each resting daughter-nucleus (figs. 10 and 11). 



As the final stages of nuclear division are being completed, the cell-body is 

 constricted round the equator, and the constriction gradually deepens to divide 

 the cell into two exactly equal parts. The spindle and polar radiations die away, 

 the last remains of the system appearing as a strand of fibres gathered into the 

 narrowing bridge between the two cells. This persists for a time as a bond of 

 union between them even after the cytoplasm has completely divided (fig. 11). 



For convenience of description and reference to the different pictures presented by the 

 nucleus in different stages of karyokinesis, it has become customary to divide the continuous 

 process into arbitrary phases. The preparatory stages up to the completed spindle are known as 

 the prophase ; the stage in which the split rods are being resolved (on the equator of the spindle) 

 into the daughter- chromosomes as the metaphase ; the stage of separation as the anaphase ; and 

 the stage of reconstruction as the tdophase. 



HISTORY OF THE SEX-CELLS. 



STRUCTURE AND DEVELOPMENT OF THE SPERMATOZOON. 



Structure of the spermatozoon. The human spermatozoon (fig. 12) is a 

 minute body possessed of a head and a long flagellum or tail. The head is conical 

 when seen in profile, but being compressed in one diameter, it is broadly oval when 

 seen in face view. At its pointed end it shows a somewhat different staining 

 reaction from the remainder ; this portion is known as the cap. The base of the 

 tail shows a thicker section, usually termed the middle piece. It includes two 

 parts, more distinctly separated from one another in some animals, the neck and 

 the connecting piece. 



The complete length of the spermatozoon is from 52 to 62 /z, the head being 

 responsible for 4 to 5 //, and the connecting piece for 6 /x, of the total figure. 



The pointed process of the head is sometimes called the perforatorium. It is 

 prolonged in some animals into a hooked projection. The limit of the cap is 

 marked by a distinct line on the head. The tail has an axial filament which is 

 prolonged through the connecting piece. In this it is imbedded in a sheath 

 derived from the cell-protoplasm, which is characterised by the presence of a 

 remarkable spiral filament. At the junction of the head and neck, and again at 

 the union of the neck with the connecting piece, there are certain darkly staining 

 granules derived from the centrosomes of the cell from which the spermatozoon is 

 developed. The tail filament is related to those which lie at the union of neck 

 and connecting piece. In some animals the tail has connected with it a very 

 delicate lateral membranous fold, which ceases a little distance short of the tip 

 (end piece). 



The picture of the spermatozoon is strikingly different from that of a typical cell, and when 

 seen in active movement produced by the lashing of the motile flagellum, the theory of the 

 early observers, to which it owes the name of spermatozoon (given to it by von Baer), seems not 

 unnatural. Kolliker (1841) first showed its true nature, by proving that the head is derived 

 from the nucleus of the cell from which the spermatozoon is formed. By recent work all the 

 various parts have been traced back to the constituent elements of the testicular cell. 



Spermatogenesis (fig. 13). The subject of spermatogenesis will be treated 

 of in the description of the testis ; but some account of the process may here 

 be given. 



The spermatozoa are derived from the cells lining the testicular tubules, which 

 Are said to multiply by amitosis, with differentiation into elements possessing 



