V. H. Blackman. 
156 
directly gives origin to the parthenogenetic egg in animals, and the 
parthenogenetic gametophyte in plants. 
The occurrence of true parthenogenesis of facultative gametes 
is well-known among the lower plants, 1 but in the case of the higher 
forms, in which the gamete has a reduced number of chromosomes, 
no case is definitely known of the development of a mature individual in 
this way. Either the organism is unable to develop with the reduced 
number of chromosomes, or the nucleus is unable to double its 
chromosomes by a process of splitting without dividing; or else a 
nuclear mechanism produced by splitting of the original chromo¬ 
somes is not sufficient for complete development. The last hypo¬ 
thesis is probably the correct one, as the peculiar behaviour of the 
unfertilised egg of the honey-bee suggests. 
The development without fertilisation of certain animal eggs 
which have formed two polar bodies has been described ( e.g . : in 
Rotifers, Daphnidae, some Insects), and these would appear to be 
cases of true parthenogenesis, but, with the exception of the eggs 
of the honey-bee (Apis niellifica), the exact cytological details seem 
never to have been worked out and there seems some doubt as to 
whether a reduction division takes place in all these cases, or 
whether the second polar body may not fuse again with the egg as 
in Artemia ; or it may be that a process similar to that described 
for the bee takes place. 
The only form in which the cytological details are throughly 
known is, as mentioned above, that of the honey-bee. Blochmann 
showed in 1889 that in this insect all the eggs form two polar bodies, 
the fertilized ones producing female and the unfertilised (partheno¬ 
genetic) ones producing males (drones). Petrunkewitsch 2 has 
investigated the development of the eggs very carefully, and states 
that in the case of the drone-egg a process of extraordinary interest 
takes place. The main body of the animal is formed from the egg 
itself, which doubles its number of chromosomes probably by a 
simple process of arrested division. The typical number (three) of 
’ Our knowledge of the nuclear changes which takes place in 
connection with fertilisation in the lower plants (except for 
such groups as Fucaceae, Dictyotaceae and Uredineae) and in 
the Protozoa is very meagre. Except for the groups men¬ 
tioned it would appear that in the majority of cases the gametes 
have nuclei which are somatic in character; the partheno¬ 
genetic development of such facultative gametes is therefore 
not surprising. The peculiar divisions before the fusion of 
nuclei to be found in such cases as Actinosphaerium, Actinophrys, 
Spirngyra and Basidiubolus would suggest that nuclear 
reduction may sometimes take place before karyogamy. 
2 Zool. Jahrb., Abt. f. Ontog. XIV. (1901) p. 573 and XVII. (1903) 
p. 481. 
