THE DEVELOPMENT OF VERTEBRATA. 215 



result is not the formation of one perfect sperm and three 

 degenerate ones, but of four perfect sperms. As to the 

 mode in which the reduction takes place there is still some 

 difference of opinion, and the nuclear structures seen are 

 not exactly the same in different species that have been 

 investigated. According to one view the number of chromo- 

 somes in the first division is unchanged, and each of these 

 chromosomes divides into two as in ordinary mitosis ; but 

 in the second division no division of individual chromo- 

 somes takes place, but half the number pass into one 

 daughter nucleus, half into the other, and thus the reduc- 

 tion is effected. According to the other view the chromo- 

 somes appear in half the original number in the division 

 spindle of the first division, and then each divides into four 

 in the two divisions. Thus the reduction of number has 

 already taken place in the formation of the spindle of the 

 first division. 



Whatever the interpretation may be, it is now known 

 that in a large number of cases the spindle of the first 

 reduction division contains half the number of chromo- 

 somes, which are characteristic of the species, and that 

 each of these chromosomes is composed of four distinct and 

 equal parts ; these chromosomes are therefore called tetrads. 

 In Ascaris megalocephala, a worm of the Phylum Nematoda 

 or Thread- worm s, which is a parasite of the intestine of the 

 horse, the somatic cells have only four chromosomes. In the 

 first reduction division of the ovum two tetrads are formed 

 (fig. 110&), and each of these consists of four simple rounded 

 granules ; a half of each tetrad passes into the first polar 

 body, and thus two dyads, or chromosomes, consisting each 

 of two granules, are left in the ovum and two in the polar 

 body. The second spindle is immediately formed without the 

 intervention of a resting stage, and the second division takes 

 place, half of each dyad passing into the second polar body. 

 The first polar body divides into two in the same way, and 

 thus there are three polar bodies and the ovum, each con- 

 taining two chromosomes of one granule each. Similar 

 divisions of two tetrads occur in the spermatocytes of the 

 Ascaris, each spermatocyte forming four spermatozoa (fig. 

 1106) . The spermatozoaof Ascaris are not of the usual form, 



