SEGMENTATION 



21 



be regarded as a secondary reversion to, rather than a persistence of, 

 a pre-teleostean habit. Possibly the reversed position of the egg 

 is to be regarded as a means of protecting its more sensitive apical 

 portion from injury by contact with the surface film of the water 

 in which it floats. 



"When fertilization takes place the most conspicuous immediate 

 result is the onset of a gradual concentration of the protoplasm in the 

 germinal disc — the disc becoming at the same 

 time more heaped up, its vertical diameter in- 

 creasing and its horizontal diminishing. 



The segmentation of the germinal disc in 

 teleostean fishes is usually of a very regular 

 and characteristic kind. It is illustrated as 

 seen in surface view by Fig. 12. The germinal 

 disc lengthens out into an elliptical shape. 

 The first furrow to appear (A) is meridional 

 and occupies the shorter diameter of the ellipse. 

 The second furrow is also meridional and in a 

 plane perpendicular to that of the first. The 

 third and fourth sets of furrows (B, C, D) are- 

 vertical and they become arranged so as to be 

 practically parallel to the first and second, with 

 the result that the blastpderm as seen in surface 

 view assumes a very characteristic arrange- 

 ment of sixteen segments arranged in four rows 

 (Pig. 12, D). 



The internal phenomena of segmentation 

 may be described from what occurs in the Trout 

 (Kopsch, 1911). In the first place it has to be 

 noted that the early furrows do not extend 

 right through the substance of the germinal 

 disc but leave a continuous basal stratum of 

 protoplasm next the yolk. The blastoderm 

 assumes a two-layered condition by the 3rd and 

 4th furrows curving round in their deeper 

 portions so as to intersect the preceding division-planes which were 

 throughout perpendicular to the surface (Pig. 13, B). Up to the 16- 

 cell stage all the segments remain connected by broad protoplasmic 

 bridges apart from the continuous basal layer of protoplasm which 

 connects the deepest cells together. 



In the 32-cell stage (Fig. 13, C) the cells of the superficial layer 

 have become completely isolated while the deep cells are still 

 connected together. With the next division the blastoderm becomes 

 three layered, the cells of the intermediate layer being derived some 

 from the superficial, some from the deep layer, as is shown by the 

 evidence of broad bridges of protoplasm which persist here and 

 there between sister cells. 



With the next division (128-cell stage, Fig. 13, D) the four-layer 



Fig. 12. — Segmentation 

 in the blastoderm of a 

 teleostean fish (Ser- 

 ranus cctrarius) as seen 

 in surface view. (After 

 Wilson, 1891.) 



