410 INTRODUCTION TO CYTOLOGY 



The type case of this category has long been the honey bee Ayis 

 mellifica, in which the eggs may develop parthenogenetically into drones 

 or, after syngamy, into females. ^^ Such a development of males with the 

 reduced chromosome number has also been demonstrated in a number of 

 other insects^^ (Fig. 229). In such males the divisions differentiating 

 the spermatids are purely equational in character; often there is but one 

 mitosis. It is probable that in certain rotifers also the male parthenotes 

 have the reduced chromosome number. 2*^ 



In addition to the foregoing cases in which the reduced chromosome 

 number is retained to sexual maturity, there are others in which the 

 number somehow becomes doubled during ontogenesis. Investigators 

 have repeatedly found that the larvae of echinoderms, insects, amphibians, 



and other animals, whether produced by 

 inducing the parthenogenetic development 

 ^S) \ \iy^ I of reduced eggs or by causing the insemina- 



..^/ Q^ tion of enucleate egg fragments, soon show 



a characteristic inability to continue very 

 far with their metamorphosis. For example, 

 _ when frogs' eggs are induced to develop by 



: iKN : ■• /\i^ '■ artificial means, mitoses with both the 

 ■•; '. fV-' .'V''^^. ') : , gametic and zygotic chromosome numbers 

 '• .•■■•..\-I*i/ .;*' can be seen in the cleavage stages and in 



.•.•.\...-- ...,.■• ■"••••■ ■ the tadpoles. The adults all have the 



Fig. 229.— Chromosomes of zygotic number.^i As in plants, there may 

 Icerya littoraiis. a, in partheno- be animals in which the somatic number is 



genetically produced male (mono- ,.,i i ..i i-ii.- 



ploid). fe, in sexually produced gametic though uot truly monoploid; but m 

 female (diploid.) {After Hughes- Tetranychus Mmaculatus, with three chromo- 



Schrader, 19306.) i • r ? • -ji ^ 



somes, and in Icerya purchasi, with two, 

 differing in size, true monoploidy cannot be doubted.-^ Nevertheless, 

 viable monoploid offspring are not, so far as known, produced by 

 animals normally diploid in both sexes. 



2. Unreduced Parthenogenesis. — The unfertilized egg develops with 

 the unreduced (zygotic) chromosome number. 



18 Petrunkewitsch (1901), Meves (19046, 1907c), Doncaster (1906, 1907), Mark 

 and Copeland (1906), Nachtsheim (1913). 



1^ E.g.: Tetranychus, Trialeurodes, Icerya, Paracopidosomopsis (Schrader, 1920, 

 1923c; Thomsen, 1927; Hughes-Schrader, 1925, 1926, 1927, 1930a6; Patterson, 1917, 

 1921; and others). In their useful review of haploidy in Metazoa Schrader and 

 Hughes-Schrader (1931) list the cases in which the reduced chromosome number in 

 the male has been demonstrated, as well as those in which the evidence is not con- 

 clusive. See also the discussion by Metz, Moses, and Hoppe (1926). 



20 Whitney (1909, 1924, 1929), A. F. ShuU (1921), Storch (1924). 



21 Parmenter (1920, 1925, 1926). See G. Hertwig (1918), Goldschmidt (19206), 

 and the discussions by P. Hertwig (1920a), Morgan (1924/), and Bosaeus (1926). 

 Doubling also occurs in Apoteitix and Paratettix (W. Robertson, 1925, 1930). 



22 Schrader (1923c), Schrader and Hughes-Schrader (1926). 



