AND OF HYBRID PIGEONS 
49 
evident that inequality in the distribution of chromatin may 
enter as a factor. 
That irregular divisions cannot account entirely for rever¬ 
sions to grandparent types is very evident in the crosses of 
brown and white doves, where the irregularities are by far too 
few to equal the percentage of reversions. There is but one 
ultimate conclusion, then, namely, that the irregularity in divis¬ 
ion of the primary spermatocytes which appears in hybrids 
between very different species, is tut an index to what occurs 
in ordinary crosses. In the latter, instead of separate spindles 
and non-fusion of chromosomes, a true union occurs, but the 
bivalent chromosomes ultimately divide in such a way that the 
respective plasmas occupy different cells. There is a separation 
of the paternal and the maUrnal chromosomes which had fused dur- 
ing synapsis. 
With regard to the question of the persistence of chromo¬ 
somes, the evidence is becoming stronger every day that these 
elements do retain their individuality. In addition to the well 
known observation of Boveri^ on the early stages of Ascaris, 
there are numerous other facts which point to the same conclu¬ 
sion. Riickert- for instance, in his study upon the fertilization 
of Cyclops, was able to follow the maternal and paternal chro¬ 
mosomes very distinctly in cleavage. Zur Strassen^ has shown 
that in the giant embryos of Ascaris which arise by a fusion of 
eggs, the number of chromosomes increases proportionately 
with the number of nuclei which unite. Again, to cite one 
more of the rapidly multiplying examples, perhaps the most 
brilliant confirmation is met with in the work of Herla^ and of 
Zoja^. Both observers have shown that in the hybrid fertiliza¬ 
tion of Ascaris, if the egg of variety hivalens is fertilized with 
the spermatozoan of variety univalens, the, three chromosomes 
1. Boveri, Th; Ueber die Befruchtung der Eier von Ascaris megaiocephala.— 
Sitzungsberichte d. Ges. Morph, n. Physiol. Munchen, II, 1887. Ueber partielie 
Befruchtung. Ibid., - IV, 1888. 
2. Ruckert, J.: Ueber das Selbstandigbleiben der vaterlichen und mutter- 
lichen Kemsubstanz wahrend der ersten Entwicklung des befruchteten Cyclops- 
Eies.—Arch. f. mikr. Anat. XLV, 1895. 
3. Zur Strassen, O: Ueber die Riesenbildung bei Ascaris-Eiern.—Arch. Ent 
wm., VII, 1898. 
4. Herla, V.: Etude des variations de la mitose chez I’Ascaride megalo- 
cephale.—Arch. f. Biol. XIII, 1893. 
5. Zoja, R.; Sullo independenza della cromatina paterna e materna nel nucleo 
dele cellule embrionali. Anat. Anz., XI, 1895. 
