34 MARGARET C. FERGUSON 



somes retain their individuality through succeeding cell-genera- 

 tions. I am, therefore, disposed to regard the tetrad-division 

 in Pinus as a true reducing division ; in this way only does the 

 complicated process just described find satisfactory explanation. 

 No positive statement can, however, be made either way, in 

 connection with this division in Pinus, until we are in posses- 

 sion of greater knowledge than at present of the origin and ulti- 

 mate destiny of chromosomes. 



Guignard ('97) expresses the opinion that the regularity of the 

 chromosomes in certain forms has been overestimated. Be that 

 as it may, I am conscious that there is recorded in this paper a 

 greater variation in the forms of the chromosomes than has been 

 described in a single genus by other writers. It has been my 

 purpose to note not only that which is in accordance, or at 

 variance, with the observations of other investigators, but to 

 give as faithful a record as possible of the conditions found in 

 the preparations studied. And may we not yet find that here, 

 in the divisions preceding spore-formation in plants, as in many 

 other instances, there is greater variation in matters of detail 

 than was formerly supposed to be the case? 



DEVELOPMENT OF THE MICROSPORE. 



The For7nat{on of the Sforc-zvall. — Hofmeister ('51) de- 

 scribed four " special " cells, each with its own wall, within the 

 pollen-mother-cell in the AhietinccB, and Juranyi ('72 and '82) 

 devoted particular attention to the formation of the wall of the 

 microspores in many Gymnosperms and Angiosperms. He 

 described the development of a wall separating the two nuclei 

 after the first division. This wall was soon absorbed and during 

 the second division the entire cell was filled with connecting 

 fibers stretching between the four nuclei. Delicate walls were 

 then laid down between the nuclei giving rise to the four 

 microspores. These dividing walls thickened and united with 

 the inner wall of the spore-mother-cell : thus a portion of each 

 spore-wall was formed from the inner mother-wall. After a 

 period of rest the outer mother-wall was burst and the " pollen- 

 cells " became free. If there is any recent literature of value 

 on this subject, I have failed to find references to it. 



