Vol.. 6, 1920 
GENETICS: BABCOCK AND COLLINS 
671 
and add to our knowledge regarding the manner of the origin and evolution 
of specific chromosome numbers. 
From a study of interspecific crosses between Nicotiana syhestris and 
several varieties of Nicotiana Tabacum, Clausen and Goodspeed^ have 
found that the Mendelian laws apply but apparently the chromosomes 
of the hybrid will function normally only when there is a union of a ma- 
jority of the chromosomes of one species. When the chromosomes of 
one gamete are mostly of a different species from those of the other uniting 
gamete the zygote either does not live or it develops abnormally. This 
seems to indicate that although they are numerically the same, the chromo- 
somes of the two species differ so much qualitatively that they cannot 
properly function together. 
Eggs of the sea urchin have been fertilized artificially with the sperm 
of another species in which the chromosomes were different in number. 
In practically all such cases the zygote would start but fail to complete 
development. It appears that the chromosome complex sets up an 
incompatible combination of genes such that the embryo cannot pass a 
certain stage of differentiation. 
The writers have found a somewhat similar case in plants in the hybrid 
between Crepis capillaris and C. tectorum. This genus belongs to that 
tribe of the Compositae having the heads composed entirely of ligulate 
flowers. C. tectorum is an annual and C. capillaris behaves either as an 
annual or as a biennial. The former has four pairs of chromosomes while 
the latter, C. capillaris, has but three pairs. Cytological studies have 
shown the hybrid to have seven chromosomes, the sum of the haploid 
numbers of the two species. 
The differences between the two species are quite marked. The achenes 
of tectorum are about 4 mm. long, are dark brown in color and retain the 
pappus quite well. The cotyledons are narrowly linear with obtuse apex, 
the shape resembling that of a sunflower seed or the cotyledons of a very 
young cucumber seedling. The first plumule leaf appears very promptly 
soon after the cotyledons have expanded and reached their maximum 
size. The plants often reach maturity in ninety days. The achenes of 
capillaris are about 2.5 mm. long, are tan or straw color, and shed the 
pappus rather easily. The cotyledons vary from broadly ovate to the 
condition where the breadth is slightly greater than the length. As in 
tectorum the first plumule leaf appears very promptly after the cotyledons 
have expanded to their normal size. 
In the hybrid, regardless of the way the cross is made, we find that 
the tectorum type of cotyledon is dominant but the seedlings are larger 
than the normal tectorum seedlings and may in that respect be said to 
show hybrid vigor. That the shape and size of the hybrid cotyledons 
growing on a tectorum mother plant would be comparable to the cotyledons 
