REDUCTION WITHOUT TETRAD-FORMATION 26$ 



tioned cell, however, does not directly become the egg, but divides 

 once, one of the products being the egg and the other the " upper 

 polar cell" (Fig. 132, F), which contributes to the endosperm-forma- 

 tion (see footnote, >Qnd compare page 218). 



In the male the two maturation-divisions are in the angiosperms 

 followed by two others, one of which separates a " vegetative " from 

 a "generative" cell, while the second divides the generative nucleus 

 into two definite germ-nuclei. In the gymnosperms more than two 

 such additional divisions take place. In these later divisions, both in 

 tRe male and in the female (with the exception noted in the footnote 

 below), the reduced number persists, and the principal interest 

 centres in the first two or maturation-divisions. Strasburger and 

 Guignard found in Lilinm that while both these divisions differed in 

 many respects from the mitosis of ordinary vegetative cells, neither 

 involved a transverse or reducing division, the chromosomes under- 

 going a longitudinal splitting for each of the maturation-divisions. 

 Further investigations by Farmer ('93), Belajeff ('94), Dixon ('96), 

 Sargant ('96, '97), and others, showed that the first division is often 

 of the heterotypical form, the daughter-chromosomes in the late-meta- 

 phase having the form of two V's united by their bases (<>). 

 Despite the complication of these figures, due to torsion and other 

 modifications, their resemblance to the ring-shaped bodies observed 

 in the first maturation-division of so many animals is unmistakable, 

 as was first clearly pointed out by Farmer and Moore ('95). 



Botanists have differed, and still differ, widely in their interpreta- 

 tion both of the origin and subsequent history of these bodies upon 

 which the question of reduction turns. According to Strasburger's 

 ('95) first account their origin has nothing in common with that of 

 the tetrad-rings, since they were described as arising by a double lon- 

 gitudinal splitting of a primary rod, the halves then separating first 

 from one end along one of the division-planes, and then from the 

 other end along the other plane, meanwhile opening out to form a 

 ring such as is shown in Fig. 133. (This process, somewhat difficult 

 to understand from a description, will be understood from the dia- 

 gram, Fig. 135, E-I^ The four elements of the ring are then distrib- 

 uted without further division by the two ensuing maturation-divisions ; 

 and the process, except for the peculiar opening out of the ring, is 



to the morphological formation of the embryo. It is a highly interesting fact that the num- 

 ber of chromosomes shown in the division of the lower of the two nuclei {i.e. the mother- 

 nucleus of the antipodal cells and lower polar-cell) formed at the first division of the 

 embryo-sac-nucleus is inconstant, varying in the lily from 12, 16, 20, to 24 (Guignard, '91, i), 

 in which respect they contrast with the descendants (egg, synergidiu) of the upper nucleus, 

 which always show the reduced number (Mottier, '97, l), i.e. in Lilium twelve. This 

 exception only emphasizes the rule of the constancy of the chromosome-number in general; 

 for these cells are destined to speedy degeneration. 



