ii4 NATURAL SCIENCE. Aug., 



hence it is greatly to be recommended, over alum carmine, for objects 

 which it is desired to cut into thin sections, but in the case of 

 objects which it is desired to mount in Mo it is liable to overstain 

 and render them opaque, and should therefore, in my opinion, be 

 avoided ; at any rate, this is so with chick embryos. 



I shall now proceed to describe the manner in which this method 

 may be applied to each of the following classes of objects, (i) to 

 cephalopod blastoderms ; (2) to chick embryos ; (3) to protozoa. 



The eggs of Sepia or Loligo are probably well-known objects to 

 the majority of zoologists, and it is not necessary for me to describe 

 them further than by stating a few essential facts about their 

 structure. Although the eggs of the two genera differ widely in appear- 

 ance they agree in being more or less oval in form and possessing a 

 relatively enormous mass of yolk, at one pole of which is placed the 

 blastoderm, which is comparatively minute at an early stage, though 

 it soon grows over the yolk. Outside the yolk comes a delicate 

 membrane, which, for the sake of description, we may term the 

 chorion, without wishing to prejudge its homologies. In the early 

 stages the chorion is in close contact with the ovum, but separates 

 more and more from it as the embryo becomes formed. Outside the 

 chorion come several gelatinous coats. In Loligo the egg is 

 embedded with a number of others in a common jelly, but in Sepia 

 each egg is separate and has its own concentric coats of jelly. 



In order to describe my method I shall take the egg of Loligo, 

 which is rather more difficult to manipulate than Sepia, on account 

 of its smaller size, though for both eggs the method is essentially 

 the same. Moreover, my remarks apply more especially to the 

 earlier stages, from the commencement of segmentation up to the 

 formation of the three layers, during all of which time the blastoderm 

 has the form of a disc or inverted saucer, its curvature being very 

 slight. After it has commenced to grow rapidly round the yolk, it is, 

 of course, impossible to prepare good surface views, for the simple 

 reason that its curved form renders it physically impossible to flatten 

 out the cup-shaped blastoderm without breaking it. Let it be supposed, 

 therefore, that we wish to obtain a series of preparations, either to 

 show the segmentation of the egg and the formation of the germ- 

 layers, or to show nuclear figures. 



The first step is to free the egg from its gelatinous envelopes and 

 leave it only surrounded by the chorion. This can be done without 

 much difficulty after a little practice. In Loligo I found it easiest to 

 remove the jelly with a pair of mounted needles in a shallow vessel 

 containing sea-water. The needles employed for this purpose should 

 be as rigid as possible, very springy needles being likely to damage 

 the structures. Now, if one needle be made to pierce the jelly, without 

 of course touching the chorion, and then be held with its point resting 

 on the bottom of the vessel, and slightly slanting, while with the 

 other hand the other needle be made to cross it iapidly in such away, 



