370 AN INTRODUCTION TO ZOOLOGY 



cleft. Thus each apparent chromosome consists of four similar 

 pieces lying closely side by side, and in this condition it may be 

 termed a tetrad. The division now proceeds as in mitosis, with the 

 result that each daughter chromosome, formed by the separation of 

 the tetrad along the line of the second split, comes to consist of a 

 bivalent chromosome of two pieces side by side. If not already 

 present the second split appears at this stage, and in either case the 

 anaphase is entered upon. Each one of the pair in the bivalent 

 daughter chromosomes is a half of the corresponding member of the 

 pair formed during synapsis. 



When this division is watched, therefore, it will be seen that each 

 daughter cell, or secondary spermatocyte, has apparently half the 

 number of chromosomes that went into the original primary 

 spermatocyte. To distinguish this type of division from mitosis it 

 is spoken of^as Meiosis or reducing division. (In older terminology 

 ordinary mitosis was said to be homceotype, since each daughter cell 

 had the same number of chromosomes as the parent cell, while 

 meiosis was designated heterotype, since parents and daughters did 

 not have the same number of chromosomes.) The subsequent 

 division of the secondary spermatocyte starts in a normal way, and 

 the bivalent chromosomes take up a position on the equatorial plate. 

 Each chromosome is already split, and does not do so again, so that 

 a half goes to each daughter cell or spermatid. There is no reduction 

 in the apparent number of chromosomes (in old terminology the 

 division is homceotype), although the chromosomes have become 

 univalent. 



Thus from each original primary spermatocyte we have produced 

 four daughter cells, each of which contains half the number of 

 chromosomes that went into the primary cell. Moreover, the 

 spermatids only receive half 'of one member of the bivalent chromo- 

 somes formed during synapsis, and not, as might be supposed, a 

 quarter of both. The spermatids, containing the haploid number, 

 undergo no further divisions, but, as the result of an alteration of 

 structure, become transformed into spermatozoa. 



While the spermatozoa of the higher animals exhibit a 

 variety of shapes and sizes they are composed of essentially the same 

 parts, and the description that follows^ is based mainly on that of 

 man or one of the higher mammals. Under moderate powers of the 

 microscope a spermatozoon appears to consist of three portions, a 

 small head, a smaller middle piece or neck, and a long fine vibratile 

 tail. Further investigations shows that it really possesses a fairly 

 complicated structure, all the parts of which are derived from pre- 

 existing portions of the spermatid. The head appears to be mainly 

 composed of the chromatin of the nucleus of the spermatid, and it is 



