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Botanists, as well as zoologists, have used the term Embryology in two senses. 
Balfour’s remarks apply to both sciences :— 
‘Strictly interpreted according to the meaning of the word, it ought to deat 
with the growth and structure of organisms during their development within 
the egg-membranes, before they are capable of leading an. independent existence. 
Modern investigators have, however, shown that such a limitation of the science 
would have a purely artificial character, and the term Embryology is now 
employed to cover the anatomy and physiology of the organism during the whole 
period included between its first coming into being and its attainment of the 
adult state.’ 
The older botanists used the term in the narrower sense. They included 
the study of the embryo-sac and the structures contained in it before the forma- 
tion of the unfertilised egg-cell, as well as the fertilisation of the latter and its 
subsequent divisions. But they did not proceed beyond the resting-stage of the 
embryo within the ripe seed. Here, as in zoology, this division is arbitrary 
and inconvenient. Accordingly, in the following remarks on the Embryology 
of Angiosperms, I include every stage in the development of the plant, from 
the first division of the fertilised egg-cell to maturity. 
Systematists, from Caesalpino onwards, have paid much attention to the 
structure of the seed, and their observations are the earliest we possess on 
botanical embryology. They were, indeed, forced to study the embryo because 
its characters are often of systematic importance. The number of cotyledons, 
for instance, is the most constant character which separates the two great classes 
of Angiosperms. Again, the endosperm is not part of the embryo, but its 
presence or absence in the ripe seed—so important systematically—determines 
the function of the cotyledons after germination, and thus influences their 
structure profoundly. In this way botanists became familiar with the structure 
of the embryo in the ripe seed before they had traced its origin from the 
fertilised egg-cell or followed its development after germination. 
The early history of the embryo was a sealed book to observers without the 
help of the compound microscope. Accordingly we find that work on the external 
morphology of seedlings preceded that on the formation of the embryo. For 
the description of seedlings we must go back to the middle of last century. 
The greatest name in this school is that of Thilo Irmisch (1815-79). His work, 
like that of earlier observers in the same field, was neglected by the succeeding 
generation owing to the rapid development of microscopic botany. For a time 
the study of anatomy eclipsed that of external morphology. 
The earliest observers to study the embryo-sac of Angiosperms with the help 
of the compound microscope were naturally attracted by the history of the ovum 
and the process of fertilisation. Little progress was made in this direction, 
however, owing to the imperfect technique of the day. The divisions of the 
fertilised egg-cell are more easily followed, as Hanstein showed in 1870. His 
classical paper is the foundation of botanical embryology in the narrower sense— 
that is, of the study of the embryo from origin to germination. 
This period in the plant’s history would seem, indeed, very well defined. 
It begins with the first division of the fertilised egg-cell—undoubtedly a natural 
epoch, for a new generation dates from it. It ends with the formation of the 
ripe seed, which is a true physiological epoch, since it corresponds with a com- 
plete change in the conditions of life. We have seen also that the morphologists 
who have dealt with the immature plant have fallen naturally into two groups, 
one ending and the other beginning their work at this very point. 
Experience, however, has shown here, as in zoology, that embryologists lose 
more than they gain by this division of their subject. It is, indeed, neither so 
simple nor so natural as it appears at first sight. 
It is not simple because the embryo is not always completely dormant during 
the interval between the formation of the ripe seed and the first steps in germina- 
tion. On the contrary, in a large proportion of Monocotyledons, and in a 
smaller but still considerable proportion of Dicotyledons, the embryo is an 
almost undifferentiated mass of meristem when the seed first ripens. It becomes 
differentiated internally and externally by degrees during the long interval 
before germination. This is sometimes called the maturation of the seed, and 
