160 EMBRYOLOGY. 



appearance gradually increases, until a slight indentation like 

 a notch is seen at some part of the margin by transmitted 

 light. This first notch fills up, but soon a similar notch occurs 

 in another spot, which is permanent. By strong reflected light, 

 if the egg lies in such a position that the white pole is directed 

 downwards, a crater-like dimple may be seen on the surface. 

 This dimple extends itself over the margin of the hemisphere, 

 diminishing at the same time gradually in depth. It is called 

 the plaited band (Faltenkranz), because a number of smaller 

 creases proceed from it at right angles. This appearance owes 

 its name to the erroneous impression that it is due to a folding 

 of the vitelline membrane, but in reality it merely depends on 

 the amoeboid movement of the germ. In fact, it is possible, 

 by close observation, to convince one's self that the furrows 

 of the plaited band are subject to active changes, for succes- 

 sive groups of them disappear, again crop up, become more 

 extensive and deeper, and then again retire. After a longer 

 or shorter time commonly one hour from the appearance of 

 the first dimple one of the folds of the plaited circle becomes 

 deeper, and spreads itself more and more towards the periphery 

 of the hemisphere, whilst the others gradually disappear. 

 Eventually a deep cruciform furrow is apparent in the hemi- 

 sphere we have hitherto had under observation, and which, as 

 previously stated, is on the opposite side to the white pole. 

 We will call this the upper hemisphere. At this time, only 

 a single shallow furrow is seen in the lower hemisphere. 

 Subsequently the furrowing proceeds somewhat more rapidly; 

 for the third, or equatorial furrow, occurs half an hour after; 

 other furrows then appear at right angles to the three first 

 formed, generally in the same succession in which the principal 

 furrows have originated; from these secondary furrows of the 

 first order proceed others of the second, and from these, others 

 of the third, and so on. The upper hemisphere divides much 

 more quickly than the lower. 



The ova of the trout are prepared as follows : The egg is 

 placed upon an object-glass between the points of a broad pair 

 of forceps, so that the blastoderm is uppermost ; the forceps 

 are held with their blades at a fixed distance from each other, 

 while the egg is pierced near its equator with a lance-shaped 

 knife. On rapidly withdrawing the knife it generally happens 

 that the blastoderm in ioto, with a large part of the tenacious 

 semi-fluid yolk, spirts out. The object must now be surrounded 

 with a ring of putty and covered. The attention of the ob- 

 server should be directed to the appearance of the elements, 

 their amoeboid movement, and to the various forms of cleavage. 

 The preparation of the ova of Batrachia is far simpler. The 

 egg is placed upon an object-glass, and as much as possible of 

 the gelatinous investment is removed with the aid of forceps 



