BY DR. KLEIN. 161 



and scissors. The vitelline membrane is ruptured by means of 

 needles, and a small portion of the escaping contents is spread 

 out in a very thin layer. If the egg is not more than three 

 days old, it can be investigated under low powers (Hartnack's 

 5 or 7) without a cover-glass. The yolk disks should be espe^ 

 cially observed, and the active movements of the pigment 

 granules with which the embryo cells are filled. Attention 

 should be further directed to the hyaline prominences which 

 the latter send out and retract, particularly after the addition 

 of a very small drop of distilled water. 



The Cleavage Cavity. The second important point, to 

 which the embryologist should direct his attention, is the 

 cleavage cavity. In the trout, this comes into existence towards 

 the end of the cleavage process. The blastoderm appears to 

 be separated from the yolk of the saucer-shaped depression by 

 a cavity which gradually increases in width and depth. The 

 blastoderm is not, however, entirely detached from the yolk, 

 but remains connected with it here and there by chains of cells. 

 These chains of cells " sub-germinal processes" may be com- 

 pared to columns by means of which the blastoderm rests 

 upon the yolk (see fig. 167). The cells of the sub-germinal 

 processes, like those of the deeper layer of the blastoderm, are 

 larger and more coarsely granular than those of the more 

 superficial layers. By degrees the cells of the sub-germinal 

 processes become separated from the blastoderm, and lie upon 

 the floor of the cleavage cavity. The elements which are found 

 in this position are characterized by their greater size, and by 

 their distinct!}' granular appearance ; they are products of the 

 blastoderm, which are either left lying on the floor of the cavity 

 when it is formed by the raising of the blastoderm from the 

 yolk, or fall to the bottom of the cavity as it increases in size. 



For the study of the formation of the cavity, that is, of the 

 elements which are to be found on its floor (the destination of 

 which we shall again have occasion to mention) and of the 

 simultaneous expansion of the blastoderm over the cavity, 

 sections are alone available. Eggs of the requisite stage (10- 

 14 days) are placed in a very dilute (one-tenth per cent.) solu- 

 tion of chromic acid, the liquid being frequently changed. 

 After a few days the eggs will have become almost black and 

 quite friable. An egg is now pierced with a lance-shaped needle, 

 and the vitelline membrane carefully torn open at one place 

 by means of sharp forceps, the rent being extended in a hori- 

 zontal direction until it describes a complete circle ; the mem- 

 brane is then removed from the upper hemisphere, which con- 

 tains the blastoderm. Thereupon the blastoderm, together 

 with the whole of the yolk of the saucer-shaped depression, is 

 separated by a sharp scalpel and placed in dilute alcohol, where 

 it may remain for any length of time. It is, however, ready 

 11 



