FER TILISA TION. 59 



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. Hertwig 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. 27, B) ; the first polar 

 body goes off with four (Fig. 27, C), and the second with two (Fig. 27, D) ; 

 leaving two. Two ' ' reducing divisions " have thus occurred. Similarly, 

 the homologue of the ovum, the sperm mother cell, contains four chromo- 

 somes in its nucleus (Fig. 27, A ). By doubling, these increase to eight 

 (Fig. 27, B'), and by division the cell forms four spermatozoa, each with 

 two. When fertilisation takes place (Fig. 28), the nucleus of the sper- 

 matozoon, with two chromosomes, unites with the reduced nucleus of the 

 ovum, also with two chromosomes ; and the number is thus raised to 

 four, the normal number in the body-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. Other cases are not so clear as that of the thread- 

 worm, but a process of "reducing division" seems to be of general 

 occurrence in the maturation of sex cells. If reducing division did not 

 occur, each fertilisation would involve a doubling of the number of 

 chromosomes. Weismann interprets the whole process as an arrange- 

 ment by which the combinations and permutations of nuclear rods and 

 their vital qualities are increased so as to give rise to new variations. 



There are, indeed, other interpretations, and the facts are difficult 

 to understand on any theory. Thus Minot, Balfour, Van Beneden, and 

 others have suggested that the polar bodies are extrusions of male 

 substance from the ovum. Biitschli, Giard, and others interpret the 

 premature division of the ovum as the survival of an ancient habit, and 

 regard the polar bodies as rudimentary or abortive ova. 



It may be possible to combine various interpretations : ( I ) the ovum 

 divides, like any other cell — like the Protozoon ancestors — at its limit of 

 growth ; (2) the extrusion does in some way differentiate the ovum, and 

 renders fertilisation possible or more profitable ; (3) the peculiar reduction 

 involved in the process makes the origin of new variations more certain. 



Fertilisation. — In the seventeenth and eighteenth cen- 

 turies, some naturalists, nicknamed " ovists," believed that 



