SUM MARY.
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 is the interval between the archesporium and the sixteen-cell stage of the sporangium. The is the interval during which the sixteen cells enlarge and tetrads are formed. The maturation -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 " archcsporial 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 primag sporocytc in its place, also sccondaly 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-peritd " 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 remain 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.
7. These methods of tetrad-formation have no significance in themselves so long as a common result is obtained. They may be considered as modifications of the same process.