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Summary. 



1. The spore maturation in Pteridophytes agrees step by step 

 with the maturation of sexual cells in animals. 



2. As in animal maturation, the process of spore-formation can 

 be divided into three periods of division, growth and maturation. 

 The division-period is the interval between the archesporium and 

 the sixteen-cell stage of the sporangium. The grozvth-period is the 

 interval during which the sixteen cells enlarge and tetrads are 

 formed. The inaUiration-period includes the two successive divi- 

 sions of the nuclei in the sixteen-cell stage, and the formation of 

 the spores. 



3. Different terms are used to designate the cells in the differ- 

 ent stages of maturation. Those of the "division-period" are 

 known as the " archesporial cells." Those of the " growth-period " 

 have hitherto been known as the " spore-mother-cells." The term 

 " spore-mother-cell " is, however, inaccurate and clumsy, and I 

 suggest the term primary sporocyte in its place, also secondary 

 sporocyte for the daughter-cells of the primary sporocyte. These 

 are the mother-cells of the spores and by their subsequent division 

 the sixty-four spores are formed. 



4. The " growth-period " is the most important stage in matu- 

 ration. It begins with a distinct enlargement of the cell. The 

 chromatin then forms a delicate moniliform spireme before the 

 nucleolus has disappeared. A much thicker spireme is subse- 

 quently formed from the moniliform thread. The thickened spireme 

 then splits longitudinally. It next breaks up into half as many 

 double spireme-segments as there are chromosomes in the somatic 

 cells ; each of these double elements forms a tetrad. 



5. Three types of tetrad-formation are found in each nucleus. 

 These may be called the "rod type," the "ring type" and the 

 " cross type." In the first type the halves of the double spireme 

 segment are completely separated; in the second, the halves be- 

 come separated in the center but remajn connected at the ends; 

 in the third type the halves become separated at the ends but re- 

 main connected at the centre. 



6. In all three types the tetrads are finally formed by a trans- 

 verse division of the halves of the double spireme-segment, giving 

 reduction in the Weismann sense. 



