46 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
The occurrence of such conditions in widely separated families 
is, however, a fact derived from comparative study, which warns 
us from inferring phylogenetic continuity. We are dealing here 
with recurrent morphological structures, called out in response to 
physiological necessities. 
Of the four megaspores formed, although as just pointed out 
their activity is indicated by their tendency to further division, only 
the upper one, namely, the one furthest removed from the chalaza, 
develops into a functional embryo-sac. No exception to this rule 
has been observed. Instead, also, of enlarging directly at the ex- 
pense of the other three by destroying and absorbing them, it 
moves into the micropylar canal, meanwhile growing at the ex- 
pense of the adjacent tissue of the ovule (jigs. 78, 79, 27, 22). In 
so doing the nucellar capping cells are destroyed. The fate of the 
remaining megaspores of the tetrad is indicated in fig. 19, 1 
which they may be seen to have undergone partial voii e. 
and to have been drawn forward toward the embryo-sac as if by 
suction. This appearance is quite constant, and is difficult to in- 
terpret in any other way. That the materials derived from them 
by their gradual disintegration and that of the other megaspores 
are used by the embryo-sac is shown by their gradual disappear- 
ance inversely to the growth of the embryo-sac. 
For some time during the development of the embryo-sac, 
however, the tetrad masses maintain their normal appearance, and 
indeed elongate considerably, but generally show signs of degen- 
eration before it is completely developed. 
At the time the embryo-sac cell commences its migration 
down the micropylar canal certain changes take place in inner lay- 
ers of the adjacent integument These consist in the rapid growth 
of the cells to many times their original volume, and the pro- 
nounced enlargement of the nuclei (jig. 73). These cells appear 
to be very active, and in some cases they fail to secrete cell walls, 
so that we find occasional cells provided with several nuclei. This 
behavior of cells, namely, their failure to form cell walls immedi- 
ately, appears to be connected with a highly nutritive function, 
and recalls the conditions in young endosperm, in the suspensor 
of some Leguminosae (Guignard, /. c.), and in the megaspore 
masses described above. Examination shows that this cell mass 
