286 THE PHENOMENON OF 



descriptions of Thwaites,* the cell-contents emerge from 

 the splitting siliceous cell-coats, and, speedily uniting 

 between the emptied shells of the mother-cells, acquire 

 the globular form, renders the assumption tolerably safe, 

 that the masses of contents unite here. No membranous 

 canal formed by an internal cell-membrane, inside which 

 the masses of contents unite, seems to exist here. Since 

 the conjugating individuals are always held together by 

 an abtindant gelatinous secretion appearing at this time, 

 it is not difficult to conceive the persistence of the pair 

 of cells in their approximated position, even without the 

 formation of membranous tubes of conjunction. The 

 reproductive cells of the Diatomacea3 formed through 

 conjugation have no seed-sleep, but, although not active, 

 pass directly, in the manner of gonidia, into vegetative 

 development. 



. a. The contents of the two cells unite without previous 

 division; in this way one gonidium is formed by two 

 mother-individuals. Thus in Himantidium.\ 



b. The contents divide in both cells before the union 

 into two masses ; in this case two gonidia are formed by 

 two mother-cells through double conjugation. According 

 to Thwaites's representation, the division of the contents 

 preceding conjugation is transverse in Eunotia, crossing 

 the direction of division in vegetative increase. 



a. The new individuals developed from the two 

 gonidia, soon surpassing the mother-cells in size, cross 

 the shells of the mother-cells. Thus in Eunotia tur- 

 gida.\ 



/3. They take a position parallel to the shells of the 

 mother-cells. Thus in Cocconema,^ Gomphonema,\\ and 

 Schizonema. 



4. The conjugating -cells unite with participation of 



* 'Annals of Nat. History,' 1847, vol. xx, pp.9, 343; 1848, ser. 2, 

 vol. i, p. 161. 



f Thwaites, 1. c., 1847, vol. xx, p. 22, f. A, 17. 

 J Ibid., t. iv. 

 Ibid,, t, xxii, f. c, 13. 

 j| Ibid., t. xxii, f. B, 1 5. 



