60 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



bodies, the first containing half, the second necessarily a 

 quarter of the nuclear material which composed the germinal 

 vesicle. The nucleus is thus reduced to a quarter of its 

 original chromatin content. It is noteworthy that the 

 second division follows close on the first without the inter- 

 vention of the "resting stage," which usually succeeds a 

 nuclear division. Moreover, there is this important differ- 

 ence between the formation of polar bodies and ordinary 

 cell division, that the number of nuclear rods or chromosomes 

 suffers reduction, whereas in ordinary karyokinesis the 

 daughter nuclei have as many nuclear rods as the original 

 cell. The extruded polar bodies come to nothing, though 

 they may linger for a time in the precincts of the ovum, and 

 may even divide. The extrusion of polar globules from 

 mature ova seems to be almost universal; but observations 

 are lacking in regard to Birds and Reptiles. Moreover, 

 Weismann and Ischikawa have shown that in all partheno- 

 genetic ova which they have examined, only one polar body 

 is formed. It is said, however, that in the parthenogenetic 

 eggs which become drones (Blochmann), and in those of a 

 moth called Liparis (Platner), two polar bodies are formed. 

 But in neither of these two exceptional cases is the partheno- 

 genesis habitual ; thus many of the eggs which the queen 

 bee lays are fertilised, and give rise to queens and workers. 



One of the most important results of recent investigations as to polar 

 bodies is due to O. Hertvvig and others. It may be briefly stated, with 

 particular reference to the ova of Ascaris megalocephala the thread- 

 worm of the horse. In one variety of this worm (var. bivalens] the 

 germinal vesicle of the ovum contains four nuclear rods, chromosomes, 

 or idants. By doubling these increase to eight (Fig. 10, B) ; the first polar 

 body goes off with four (Fig. 10, C), and the second with two (Fig. 10, D) ; 

 leaving two. Two "reducing divisions" have thus occurred. 

 Similarly, the homologue of the ovum, the sperm mother cell contains 

 four chromosomes in its nucleus (Fig. 10, A 1 ). By doubling these 

 increase to eight (Fig. 10, B 1 ), and by division the cell forms four 

 spermatozoa, each with two. When fertilisation takes place, the nucleus 

 of the spermatozoon, with two chromosomes, unites with the reduced 

 nucleus of the ovum, also with two chromosomes ; and the number is 

 thus raised to four, which is the normal number in the cells of this 

 variety of Ascaris megalocephala. There is thus a striking parallelism in 

 the history of the two nuclei which unite in fertilisation ; both have been 

 subjected to reducing divisions. If this did not occur, each fertilisation 

 would involve a doubling of the number of chromosomes. Weismann 

 interprets the whole process as an arrangement by which the corn- 



