ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



491 



with a condensing lens in the lower end. The two latter have a hinged 

 shade which can be placed in different positions in front of the lens 

 according to the illumination required. 



Fig. 83. 



Fig. 84. 



Gerlach's Embryoscope.* — The embryoscope, devised by Dr. L. 

 Gerlach, supplies a great and long-felt desideratum in experimental 

 embryology. It is a mechanism for closing hermetically a circular 

 opening, made with a trepan, in the shell of the hen's egg ; and it serves 

 the purpose of a window, through which the living embryo may be 

 directly observed, and its development followed from day to day. 



The instrument consists of two parts : — (1) A mounting-ring to be 

 firmly cemented to the egg-shell. (2) A key-piece with glass front, 

 which screws into the ring and closes it air-tight. 



Fig. 85 represents the embryoscope in perspective, and fig. 86 in 

 section. The metallic mounting-ring is 1| mm. thick, and has a lumen 

 2 cm. in diameter. The lower edge Ar is bevelled and saddle-shaped so 

 as to fit the equatorial surface of the egg, while the upper edge is flat. 

 From the outer surface of the ring two square-cornered bars Z project 

 in opposite directions. On its inner surface, a little above the lower 

 edge, is a diaphragm Md with an opening 13 mm. in diameter. Rest- 

 ing upon this diaphragm, and corresponding with it in size and shape, is 

 a second diaphragm of thin wax-cloth Wd, which serves as a packing- 

 washer for the key-piece. 



The key-piece of the embryoscope consists of a low metallic cylin- 

 der, closed by a disc of glass G, which represents the window that is to 

 cover the artificial opening in the shell. The upper part of the cylinder 

 expands peripherally to form a rim with a milled edge Vs. This rim has 

 two notches E opposite each other, into which fit the arms of a small 

 wrench, by the aid of which the key-piece can be tightly screwed down. 



* Anat. Anzeig., ii. (1887) p. 583 (2 figs.). 



