FORMATION OF THE MEGASPORES 227 



somes as just described, but that the thread remains single and 

 segments into end-to-end pairs of chromosomes, the maternal 

 member of each going to one pole and the paternal member 

 to the other, the final result being the same in both cases. 



Formation of the Megaspores. — The embryology of the 

 flower of Silphium has been worked out by W. D. Merrell, and 

 his work will be followed in tracing the production of the mega- 

 spore, as was done in our study of the microspore. 



Early in the development of the ray flowers, which alone are 

 fertile in Silphium, when the ovule has reached the stage shown 

 in Fig. 131, A, o, the archesporium appears at its apex as a single 

 hypodermal cell, D, a. As the archesporium elongates, a single 

 layer of cells surrounding it and constituting the nucellus, grows 

 out with it (B and Ca, and E). Then the upper end of the 

 ovule becomes inverted, Ca and C&, and tissue around the base 

 of the nucellus grows forward and forms the integument, Ca 

 and C&. 



When the archesporium enters upon its first nuclear division 

 (Fig. 131, F) the number of chromosomes sent to each pole 

 is 8, which is one-half the number in the ordinary vegetative 

 divisions. The reduction in the number of the chromosomes 

 here points to the archesporium as the homologue of the grand- 

 mother cells of the microspores. Each daughter cell of the 

 archesporium divides once, yielding a row of four cells or mega- 

 spores within the nucellus (Fig. 131, G). Only the uppermost 

 megaspore, m, proceeds to grow. As it elongates it crowds the 

 three disintegrating megaspores toward the micropylar end 

 (Fig. 131, H) where they are soon lost sight of. 



Details in Division of Megaspore Grandmother Cell. — The 

 details in the division of the megaspore grandmother cell are es- 

 sentially like those in the microspore grandmother cell, and we 

 assume that here also in respect of a group of characters borne 

 by a single chromosome two of the four megaspores descended 

 from the grandmother cell would be of paternal and two of 

 maternal origin, as shown in I, Fig. 126. 



