Asexual Mode of Auxospore-formation 



OF 

 Arachnoidiscus Ehrenbergii Bail. 



By 



K. Yendo and K. Akatsuka. 



In January, 1910, we had an opportunity of observing the 

 mode of auxospore-formation of Arachnoidiscus Ehrenbergii 

 Bail. The spores have been formed asexually, each from a single 

 frustule. In the essential points, the process of formation ac- 

 cords satisfactorily with that has been already observed by 

 Smith, Schutt, &c, on Melosira. But as it has not yet been 

 known in the above mentioned species, we shall give a brief 

 account on it. 



The frustules of Arachnoidiscus Ehrenbergii Bail, attach on 

 a substratum with the upper surface of epitheca, on various 

 algae, especially, in our case, on the fronds of Rhodomela 

 Larix. They are discoidal, and have trabecular ridges in radial 

 directions on the inner surfaces of both valves. The ridges con- 

 verge towards the central nodule, leaving there a circular area. 

 There are ring-shaped shelves with a narrow room between the 

 valve, around the nodule and along the margin, spanned by the 

 ridges. The outer surfaces are sculptured with minute but 

 square spots in the area between two adjacent trabecular lines. 

 For details concerning this matter we recommend the readers 

 to refer to Schmidt's Atlas der Diatomaceen Kunde, Taf. 6-8, 

 Fig. 1. 



A frustule, within which the auxospore is going to be form- 

 ed, becomes extremely rich in chromoplasts and increases in its 

 height as if in the way of division. The spore grows by de- 

 grees lifting up the hypotheca, and as soon as the valves are 

 parted, the spore swells up spherically with the two polar re- 

 gions still remaining within the valves. In this respect it 

 resembles very much to the spore-formation of Melosira \arians 



