[21] EMBRYOGRAPHY OF OSSEOUS FISHES. 475 



reception of the spermatozoon, such as lias been described by Kupflfer 

 and Benecke in the egg of the hitnprey. This striuitiire is represented 

 in Fig-. 7, PI. I, at pp. in an egg of the cod, in which the formation 

 of the germinal disk has not yet been completed. Eejieated observa- 

 tion convinced me that I was not looking at an accidental feature, but 

 that it was constant during this and somewhat later stages of develop- 

 ment antecedent to segmentation. The actual ingress of the spermato- 

 zoon into the ovum I have never witnessed, although the cod's egg is 

 one of the best adapted of all hsh ova for this purpose on account of 

 its transparency and small size. If Fol's compressor is used, experiments 

 may be very readily carried out undei the microscope, and if the upper 

 and lower plates of the compressor are kept far enough apart so as just 

 to allow the eggs to remain free and mobile within a few drops of water, 

 encircled by a ring of block tin or hard rubber, clamped by the cover, 

 the eggs will always arrange themselves in one position, with the germ- 

 inal disk downwards and the yelk uppermost. This peculiarity enables 

 one to see only the lower face of the disk through the large transparent 

 yelk above it when the tube of the microscope is ]daced vertically, or 

 its edge, or in aptic section, when the tube of the microscope is placed 

 horizontally, with the stage and compressor upright. To sec the upper 

 surface of the germinal disk of the live Ggg it is most convenient, in 

 fact necessary, to have an invertiug prism attached to the microscope, 

 into the mounting of which the objectives may be screwed, so as to view 

 the eggs from below. Cachet's inverted microscope, used in cheuiicul 

 investigations, would answer well for this purpose. The sketches which 

 I have made were obtained from living eggs treated in this way, with- 

 out compression while confined within the area of a hard rubber or 

 metal ring, which served to hold the water and eggs in place when the 

 cover of the compressor was screwed down. Attemi^ts made to wit- 

 ness the entrance of the spermatozoa by the help of the above described 

 apparatus, using very dilute mixtures of milt with water, were not 

 successful. The jjroper mode of procedure, in order to demonstrate the 

 changes by histological methods, would be to take a batch of ova fresh 

 from the ovary and divide them into two lots. Impregnate the one lot 

 and allow the other to remain unimpregnated. Then take of both a 

 series of specimens at intervals of two or three minutes and place them 

 in a dilute chromic acid solution to fix the nuclear and other protoplas- 

 mic changes, so as to afterwards facilitate staining and the preparation 

 of sections, and the satisfactory study of all the changes which the 

 luiclear matter of the germ has undergone until the time of the first 

 segmentation. A similar series of the unimpregnated ova would throw 

 some light upon the history of the process of the migration of the 

 nucleus from the center of the egg, if taken in connecf.on with the 

 investigation of the mature and functionally active ovary with its pro- 

 ducts in different conditions of maturity. 

 The complete disappearance of the germinative vesicle from ova in 



