Emhryonic Excretory Organs of Ceratodus. 323 



The mesonephros is of the usual type. It arises as 

 curved bands of cells in the mesoblast. In each of these 

 a lumen appears, and the inner end of this expands (Bow- 

 man's Capsule), while the tubule becomes long and con- 

 voluted, and opens at one end into the body-cavity, while 

 the other end connects with the segmental duct. 



XXV. Simpler Methods in Crystallography. Part I. Stereo- 

 grams. By J. G. GooDCHiLD, H.M. Geol. Survey, 

 F.G.S., F.Z.S. [Plates X., XI., XI L] 



(Read 18th April 1900.) 



1. In the universities and other educational institutions, 

 where much attention is devoted to the mathematical 

 sciences, there may be said to be often a need for subjects 

 of a sufficiently varied character to which these mathe- 

 matical studies can be practically applied. The higher 

 branches of physics furnish ample scope for such work in 

 one direction, while in another the subject of Astronomy 

 affords excellent opportunities for mathematical work of a 

 somewhat different kind. For the same reason (though occa- 

 sionally for the sake of the subject itself) a few students are 

 now and then induced to apply their mathematical knowledge 

 to the working out of problems in Crystallography. It results 

 chiefly from this cause, that most of the treatises on that 

 branch of science have been penned by those who have 

 approached the subject from a purely mathematical starting- 

 point. In consequence of this (which is, perhaps, almost 

 unavoidable under the circumstances). Crystallography has 

 usually been treated in such a manner that those workers 

 who do not happen to possess the special knowledge 

 necessary for the right understanding of these treatises, find 

 themselves debarred at the very outset from doing any 

 practical work in this really interesting branch of science. 



2. Several years' practical experience in the determination 

 of crystalline forms, and in the clinographic and other 

 delineations of the actual crystals so determined, has fully 

 confirmed a view that I have long held, that much useful 



