Micros/pore formation 



111 



MICROSPORES. Reproduction by the formation of small spores termed 

 microspores has been described as occurring in a number of diatoms, The 

 existence of minute spores was suspected by Kitton ('85), who stated that 

 they were so small as to readily pass through filter papers. Castracane ('97) 

 has gone so far as to state that the normal method of reproduction of diatoms 

 is by microspores, and that multiplication by division, although very common, 

 is the exception rather than the rule. The latter statement, however, cannot 

 be accepted in view of the fact that all observations tend to show that the 

 normal method of multiplication of diatoms is by cell- division. This is still 

 further emphasized by the numerous culture-experiments 

 of Miquel ('92 '98), and of Allen & Nelson ('00), in 

 which the diatoms multiplied only by cell-division. Gran 

 has described the formation of microspores in Rhizosolenia 

 styliformis ('02) and Chwtoceras decipiens ('04) by the 

 successive division of the protoplast to form sixteen spores. 

 Ostenfeld ('10) has confirmed Gran's observations in the 

 latter species, although he found in the samples he 

 examined that division of the nuclei may not be at once 

 followed by division of the cytoplasm. 



Karsten ('04) has given some interesting observations 

 on the formation of microspores in Corethron Valdivite, one 

 of the centric diatoms of the antarctic seas. In this case 

 the protoplast divided by successive binary fissions to form 

 as many as 128 globular cells each surrounded by a proto- 

 plasmic membrane (fig. 81 A and B). Simultaneous division 

 of the nuclei occurred during the formation of these small Fig. 81. Formation of 

 cells, and in the division from 16 to 32 nuclear spindles were 

 found, thus indicating that all the divisions are probably 

 mitotic. Eventually the small cells escape and hang in 

 clusters entangled among the spines of mature individuals. 

 These cells are gametes, which fuse in pairs with those 

 from other cells to form zygotes. The latter increase in 

 size and divide into two daughter-cells, each of which 

 possesses two nuclei. Only one nucleus remains as the 

 daughter-cell becomes further organized into the diatom- 

 cell, the other disappearing. The protoplast of the developing cell escapes 

 from its first wall, and then expands to form a complete Corethron. The new 

 siliceous wall develops gradually and the normal length is attained by the 

 elongation of the girdle-bands. The ' tetrad-division ' here follows conjuga- 

 tion (as in the Desmidiacea;) and is a twice-repeated mitosis of the 

 zygote-nucleus, in contrast to the 'tetrad -division' which preceded conjugation 

 in the formation of the auxospores of Rhopalodia yibba. 



microspores in Core- 

 thron ValdivisB Kar- 

 sten. A, division of 

 protoplast into 32 ; 

 B, further stage of 

 division into 128 

 small rounded por- 

 tions, each of which 

 becomes a micro- 

 spore (in this case 

 a gamete). x 334. 

 (Somewhat modified 

 from Karsten.) 



