878 
cell reaches a length of about one-eighth 
of an inch. 
“The most usual shape of the free cells 
is the spherical, and cells forming a part 
of a tissue are more or less rectangular 
in form. 
“When first formed, the cells of the in- 
dividual animal or plant are very much 
alike, but as one examines the cell furth- 
er from the regions where actual cell 
multiplication is taking place, it is seen 
that the originally similar cells are be- 
coming very unlike. In the higher 
plants, the outer cells become different- 
iated into protective tissue, the innermost 
into conductive tissue, others into as- 
Similative tissue, and still others become 
reproductive cells. In higher animals, 
Similar differentiations take place, cells 
which finally become so different as those 
which form nerves, muscles, glands and 
even teeth, having been practically alike 
in the beginning. Among the unicellular 
organisms, there is often remarkable dif- 
ferentiation and division of labor, the 
Single cell performing the functions of 
locomotion, securing food, digestion, as- 
similation, etc. Such differentiation and 
the causes which lead to it are among 
the most important cytological problems. 
“Tt is a remarkable fact, that while 
undergoing nucelar division, the cells of 
plants and animals strikingly resemble 
each other, even in the behavior of the 
most remote constituents of nucleus and 
protoplasm. 
“This must mean that animals have 
been derived from plants, or that struc- 
tures of amazing similarity have arisen 
independently in animals and plants. 
Fertilization 
“Fertilization is one of the most im- 
portant problems of cytology. While new 
individuals without fertilization, even in 
the more highly organized animals, may 
occur occasionally (by parthogenesis, 
chemical stimulus, vegetative multiplica- 
tion, etc.) such cases are so rare, that 
fertilization is assumed to occur, unless 
its absence is clearly established. To the 
eytologist, fertilization consists in the 
union of definitely organized male and 
female elements. The cytological details 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
of fertilization of plants and animals are 
essentially alike. Fertilization both in 
plants and in animals is preceded by a 
reduction in the number of chromosomes, 
so that the number of chromosomes 
found in the male nucleus or in the fe- 
male nucleus is just one half the num- 
ber found in the body cells of a given 
plant or animal. Consequently, when the 
two sexual nuclei fuse during fertiliza- 
tion, the number of chromosomes which 
characterizes the body cells is restored. 
The complicated details of the process by 
which this reduction in the number of 
chromosomes is effected is essentially 
alike in plants and animals. Those cytol- 
ogists who have investigated most 
thoroughly the phenomena of fertiliza- 
tion have come to the conclusion that 
heredity is referable to a definite cyto- 
logical basis. 
A. Cytological View of Heredity 
“Almost without exception, cytologists 
have believed that chromatin is the 
physical basis of heredity. The reasons 
for this belief are briefly as follows: The 
male and female parents are about equal- 
ly potent in transmitting characters to 
offspring; an equal amount of chromatin 
and an equal number of chromoses are 
contributed by each parent; nothing but 
chromatin is contributed equally by each 
parent. There is usually a great differ- 
ence in size between the male and the 
female germ cells. The sperm cell 
(spermatozoan) of the ostrich, is almost 
invisible to the naked eye, while the egg 
is as large as a cocoanut, and such diff- 
erences in size are usual both in plants 
and animals. The egg contains a large 
amount of protoplasm and various food 
stuffs and in many cases even that little 
is left outside at the time of fertiliza- 
tion, only the nucleus entering the egg. 
Hence, protoplasm and foodstuffs do not 
transmit hereditary characters. While 
the male nucleus is usually the smaller 
at the time of its entrance into the egg, 
it increases in size so that at the time of 
fusion the sex nuclei are alike in size. 
“The organization of embryos and ma- 
ture organizations from eggs is a cyto- 
logical problem which has not yet been 
