Mar. II. 1918 Sterility in the Strawberry 647 



the periphery remaining in that position and those which have one end 

 attached to the nucleolus assuming a position alongside of it (PI. B, 

 II, 14). Occasionally one pair may be attached both to the nucleolus 

 and to the periphery ; when there is evidence of considerable force exerted 

 by the pair in contraction (PI. B, 11), The pairs continue to contract 

 (PI. B, 12, 13), forming various figures which have often been described 

 in other forms, but very few circles have ever been seen at this time. 

 The contraction continues until the typical diakenesis stage is reached 

 when it is often difficult to distinguish between the two univalent 

 chromosomes of a pair (PI. B, 14, 15). Apparently they often fuse, 

 as in the multipolar spindle stage they sometimes appear as single entities. 

 At diakenesis 26 chromosome pairs can readily be counted. At this 

 stage in Minnesota No. 3, of 22 counts made, 19 showed definitely 26 

 chromosome pairs and three others showed, respectively, 24, 25, and 27. 



A very similar condition to the diakenesis of the pollen mother cells is 

 shown in the prophase of the tapetal cell divisions. Here, however, in 

 place of the 26 pairs of chromosomes 52 pairs appear arranged about the 

 periphery of the nucleus. Five counts made at this time showed in 

 three cases 52, and in two 50 and 54 pairs, respectively. 



Heterotypic division. — Diakenesis in the pollen mother cells is of 

 somewhat long duration, but the period between it and the metaphase 

 of the heterotypic division is extremely short, usually not more than two 

 or three multipola'r spindles appearing in a loculus simultaneously. The 

 small oval chromosomes now arrange themselves on the equatorial plate. 

 Whether there is any definite order or arrangement could not be deter- 

 mined, as the chromosomes appear identical. They are arranged close 

 together and, while their number can not be readily determined, 26 have 

 been counted on one plate and 24 on another. 



The chromosomes are then gradually pulled apart and drawn to the 

 poles. No irregularities in cell division or extrusion of chromatin matter 

 have been seen during this process. The daughter chromosomes show 

 only slight evidence of fission for the following division. The disk- 

 shaped daughter nuclei are soon formed (PI. C, i), and directly after 

 prepare for the second meiotic division. 



HoMEOTYPic DIVISION. — The two spindles of the homeotypic division 

 may be parallel to one another or their axes may be at right angles. 

 The metaphase of the division is also characterized by great uniformity, 

 the daughter chromosomes separating and advancing toward the poles 

 simultaneously. After separation they could be readily counted and 

 showed in 7 counts 26 chromosomes (PI. C, 2). 



The daughter nuclei are soon formed, and walls are laid down between 

 them, dividing the cytoplasm evenly. The cells gradually split apart, 

 separating the four microspores and allowing the entrance between 

 them of the viscous material which has up to this time surrounded either 



