470 THE AMERICAN NATURALIST. [VOL. XXXIX. 



somes in sporophyte and gametophyte for more than fifty forms. 

 This list may be found in Coulter and Chamberlain's recent 

 text-book, The Morphology of the Angiosperms, 1903, p. 81. 

 Farmer's accounts of the number of chromosomes in the Hepaticae 

 have been confirmed and extended by myself (Davis, '99, :oia) 

 and by Moore (:O3). The more recent literature, especially as 

 it concerns the events of spindle formation in the mitoses char- 

 acteristic of sporogenesis has been treated in our account of the 

 spore mother-cell (Amer. Nat., vol. 38, p. 725, Oct., 1904). 



There are two chief periods in the processes of sporogenesis 

 as illustrated in all groups above the thallophytes : ( i ) a growth 

 period and (2) a period of cell division. In the growth period 

 the spore mother-cells become differentiated from the general 

 sporogenous tissues through a great increase in the amount of 

 protoplasmic material. At some time in this growth period the 

 nucleus of the spore mother-cell exhibits the phenomenon of 

 synapsis, a very characteristic event, recognized by the very 

 much contracted condition of the chromatin network in the 

 interior of the nucleus. Synapsis is believed to hold fundamental 

 relations to reduction phenomena as the time when chromosomes 

 unite with one another in pairs. The period of cell division fol- 

 lows synapsis and is characterized by two mitoses in the spore 

 mother-cell, the second following immediately upon the first, and 

 a segmentation of the protoplasm, sometimes by two successive 

 divisions, and sometimes by a simultaneous cleavage, into four 

 spores. The two mitoses present certain peculiarities in the 

 structure and behavior of their chromosomes which are unlike 

 the events of typical mitoses. The first is known as the hetero- 

 typic and the second as the homotypic mitosis. These peculiar- 

 ities have been recognized for a long time and have furnished 

 the subject of much investigation and contradictory explanations. 

 They were briefly described in Section III (Amer. Nat., vol. 38, 

 p. 740, Oct., 1904) but recent studies of Farmer and Moore 

 (: 03, : 05) have opened again a discussion which seemed closed 

 at that time. The details of synapsis and the heterotypic and 

 homotypic mitoses will be taken up under the caption, " Reduc- 

 tion of the Chromosomes." 



Contrary to a statement in Section III of these studies (Amer. 



