PROCESS OF DEVELOPMENT IN AVIAN OVA. 510 



The proper method of imbedding is to make a little paper case, which 

 is then half filled with a mixture of wax and oil. 



When the mixture has so far stiffened in the paper case that a pre- 

 paration laid upon it will not sink in it, the specimen is placed in 

 the desired position, and is again covered with the fluid mixture till 

 the case is full. As soon as this second mass begins to stiffen, the 

 position of the specimen is carefully noted, and when the stiffening is 

 completed, the direction in which the section is to be made is marked. 

 The proportion of the oil to the wax should be so chosen that the 

 consistence of the mixture corresponds to that of the specimen and 

 the convenience of the operator. 



I cannot allow this opportunity to pass without expressing my 

 astonishment at the obstinacy with which even excellent micro- 

 scopists overlook the advantages afforded by such a mixture as the 

 above. The mixture of two bodies of such different consistence as 

 wax and oil enables us to obtain all grades of consistence between 

 those two extremes. For extremely delicate embryological specimens, 

 such advantages are simply inestimable. 



I shall here incidentally remark, that where the specimens, as in 

 the case of the ova of Frogs, enclose cavities in their interior, these 

 must be opened to permit them to be filled with the mass. It is only 

 in those instances .where the specimen and the mixture of wax and 

 oil are everywhere in complete and accurate contact, that we can 

 expect to obtain good preparations. 



The paper case should be sufficiently large to enable the operator to 

 hold it securely. The knife should also be large, as sharp as possible, 

 and on one side at least ground flat. When in use, its upper surface 

 should be wetted with oil of turpentine. On completing the cut, the 

 section should be floated off upon the slide by the aid of a little oil of 

 turpentine. It can then be preserved in the usual way with oil of 

 cloves, Dammar balsam, and a paper cushion. 



So far as sections of ova hardened in chromic acid permit us 

 to speak positively in regard to the real form of the parts, 

 the perfectly mature germ has the shape of a biconvex lens, 

 which is somewhat concave at one pole, that, namely, which in 

 the natural position is the inferior. Its diameter amounts to 

 about half a millimeter ; its thickness is about 0*05 of a milli- 

 meter at the concave, and O06 at the biconvex points. In such 

 a sharply defined body the germ has to be sought. The vitel- 

 line membrane is above it, and beneath is a finely granulated 



