216 



ZOOLOGY. 



with a long vibratile tail, but are conical and, like those of 

 the Arthropoda, motionless ; the broader end of the cone 

 contains the nucleus. The tetrad does not always consist 

 of four simple granules. In some cases, e.g. the spermato- 

 genesis of Salamander and of Helix and the pollen forma- 

 tion of the Lily, after a long period of preparation or 



Fig. 110 a. Reduction divisions (maturation) of the ovum in Ancnrit 

 eepfuila, variety with four chromosomes in somatic cells. A, the ovum with the 

 spermatozoon, Spz., entering it; the nucleus of the ovum has two chromosomes, 

 each in the form of a tetrad. B, the nucleus of the ovum has travelled to the sur- 

 face and the tetrads are dividing. C, the division is complete, and the first polar 

 .body Pbl is formed, it contains, like the ovum, two dyads. D, the dyads in the 

 ovum preparing for a second division. , the dyads dividing. F, division complete, 

 second polar body Pb2 formed, first polar body divided into two. Pn$, male 

 pronucleus. 



prophase the convoluted chromatin thread becomes divided 

 into lengths forming the chromosomes, and each of these 

 splits longitudinally, the two halves separating in the 

 middle and remaining connected by their ends, so forming 

 a ring without any other division ; it has been suggested 

 that the two halves of the ring are really separate, chromo- 



