752 THE AMERICAN NATURALIST.  [VoL. XXXIII. 
This is exactly parallel with what takes place in the growth of the 
shark’s egg at the expense of its follicle cells, and reminds us also of 
similar processes in insect eggs. 
3. A maturation period, in’ which the central cell is prepared for 
fertilization. This consists in a very unequal cell division, resulting 
in the formation of a small canal cell which rests as a small cap on 
the peripheral end of the large, oval egg-cell. 
Pollination occurs in June or July, and is shortly followed by the 
formation of the pollen tube at about the same time that the arche- 
gonium is being differentiated. The changes which take place in the 
pollen tube leading to the formation of motile spermatozoa cover a 
period of two months or more, at the end of which period fertilization 
is accomplished, in September or October. 
A pollen grain is spherical in form, containing three cells placed 
in a row, namely, a large “embryonal cell” and two small flattened 
“ prothallium cells.” The embryonal cell plays the principal part in 
the formation of the pollen tube, occupying a position near its grow- 
ing tip, the prothallium cells meanwhile remaining quiescent at the 
opposite end of the tube. Having formed the pollen tube, which 
now lies imbedded in the endosperm, the embryonal cell seems to 
have performed its principal function, and it subsequently dis- 
integrates. 
The larger prothallium cell, the one which was next to the em- 
bryonal cell in the pollen grain, may be regarded as a primordial 
germ cell; it now begins to develop, passing successively through 
stages of division, growth, and maturation, corresponding to those 
which occur in animal spermatogenesis. In the first of these stages 
nuclear division occurs without division of the cytoplasm, one of the 
nuclei being thrown out into the pollen’ tube as the “ Stielzelle.” 
Early in the second, or growth period, centrosomes appear in the 
germinal cell (spermatocyte, to use the terminology of animal sper- 
matogenesis) on opposite sides of the nucleus. These are very large 
deeply staining bodies, which persist throughout the subsequent 
development and have an interesting fate. “The spermatocyte attains 
a‘diameter of about 0.14 mm., its large nucleus being about 60 
mikra in diameter, and the centrosomes 10-15 mikra (!) in diameter. 
The centrosomes, except for a few vacuoles which they contain, are 
solid structures, as is shown by the fact that they can be broken into 
fragments by pressure. Around them are seen faint cytoplasmic 
radiations. 
Maturation is secommplished by the completion of the division 
