1884.] 



MICROSCOPICAL JOURNAL. 



109 



not reaching distinctness and definite- 

 ness of outline make the ornamenta- 

 tion of different species. Thickened 

 dots are also sometimes met, and b}' 

 the deeper pink color which marks 

 them are easily distinguished from 

 and contrasted with the areolae with 

 which we have chiefly had to do. 



The extreme of irregular sculptur- 

 ing is met with in such forms Ste- 

 phatiogonia Dariica in the Jutland 

 cementstein, where all resemblance 

 to the more ordinary marking of dia- 

 toms is lost. 



As a summary of the results of my 

 numerous observations of which typi- 

 cal examples have been given, I would 

 say that we find the following conclu- 

 sions warranted : 



1 . The diatom shell is usually 

 formed of two laminae, one or both 

 of which may be areolated, and may 

 be strengthened by ribs which have 

 been described both as costae and as 

 canaliculi. 



2. The normal form of the areolae 

 is a circle, and these when crowded 

 together take a hexagonal and sub- 

 hexagonal form. 



3. The areolae are properly pits or 

 depressions in the inner surface of 

 one of the laminae, so that when two 

 laminse are applied together the exte- 

 rior surfaces of the shell thus formed 

 are approximately smooth and the 

 cavities are within. 



4. The apparent thickening on the 

 exterior of the lines bounding the 

 areolae in some species, as Etcpodis- 

 cus argus^ etc., is not in contraven- 

 tion of, but is in addition to, the form- 

 ation above described. 



5. However fine the dotted marking 

 of diatom valves may be, the evidence 

 from the color of the spaces between 

 the dots and of the dots themselves 

 supports the conclusion that they fol- 

 low the analogy of the coarser forms 

 in which both fracture and color are 

 found to prove that the dots are are- 

 ola and the weaker plap^S in the shell. 



Microscopical Techiiic. 



V. MOUNTING IN GELATINOUS AND 

 RESINOUvS MEDIA. 



As these articles continue we re- 

 ceive occasional inquiries and sugges- 

 tions which indicate that they are of 

 interest to many readers. To the 

 writer such communications are of no 

 little assistance, for they show what 

 kind of information is desired. We 

 cannot undertake to answer so many 

 inquiries by letter, but the subjects are 

 duly noted, and will receive attention 

 in their proper places. 



Mounting in gelatinous media. — 

 Under this head are included such 

 media as glycerin-jelly, Farrant's me- 

 dium, Deane's medium, etc. All of 

 these consist essentially of various 

 proportions of glycerin, gelatin, and 

 water. They are useful for mounting 

 delicate tissues, being intermediate be- 

 tween water and Canada balsam in re- 

 fractive power, and they require but 

 little heat in use. We would be glad 

 to see them more generally used in- 

 stead of balsam. In our own work 

 but little use has been made of them, 

 for the reason that we have preferred 

 mounting in fluid media — water or 

 glycerin, or mixtures of the tw'o. 

 However, every one has his prefer- 

 ences, and it seems very strange that 

 methods which prove entirely suc- 

 cessful with one person do not suc- 

 ceed with others. It is natural, there- 

 fore, that while we have preferred 

 fluid mounts, many others will prefer 

 glycerin-jelly. The results are about 

 the same, although in using fluids the 

 preparer has a wider range of selec- 

 tion in choosing the mounting media 

 for different objects. This subject 

 will be brought up again in its proper 

 place. We have now to describe the 

 method of mounting in glycerin-jelly, 

 which ma}' be taken as the type of 

 these gelatinous media. 



The more delicate objects, if trans- 

 ferred from water directly to the me- 

 dium, would become shrunken and 

 the parts more or less distorted. 

 Hence it is advisable to place them 



