OBSERVATIONS ON DEVELOPMENT OF COMMON TROUT. 123 



more or less distinctly fibrillar structure especially well shown 

 in the part next to the arehiblast ; (c) it contains a great 

 number of well-defined vesicular nuclei,, many of which con- 

 tain a nucleolus; they (nuclei) are most numerous in the parts 

 near the edge of the archiblast, and are of different sizes and 

 shapes — chiefly elongated in the superficial parts — and appear 

 generally to be disposed in groups, forming chains of bodies 

 more or less pressed against each other. To these nuclei we 

 shall return more minutely hereafter. 



[Note. — The fibrillar structure just referred to is of such a 

 character that the apparent " fibrils " run in a direction 

 parallel to the free surface. Balfour mentions the presence, 

 in osmic acid specimens, of a fine network of fibrils *' in any 

 part of the fine granular yolk around the blastoderm," that 

 is, in the portion comprising " the nuclei of the yolk."] 



Next in order is a blastoderm twelve days old, which in 

 vertical section exhibits a distinct subgerminal or segmenta- 

 tion cavity. This is seen to be situated excentrically in con- 

 formity with the assertion of Oellacher. The archiblast is 

 thicker in the periphery than in the centre. The former is 

 not of uniform thickness, for at one side it is much thicker 

 than at the other (see fig. 4) ; this latter corresponds to that 

 part of the periphery towards which the segmentation cavity 

 extends. The central thinner part of the archiblast is several 

 cells deep ; they are more or less polyhedral, and contain 

 one or two nuclei. At the periphery there exist underneath 

 these, in addition, layers of cells, which are more opaque, 

 containing numerous yolk granules ; they are of different 

 sizes, and possess not unusually a nucleus in the state of 

 division or two or more nuclei ; also to the lower surface of 

 the thin part of the blastoderm is attached one or the other 

 cell of this character. 



The parablast is seen to have increased considerably in 

 extent (see fig. 4). Although under a low power only the 

 thicker portions of it are discernible, yet the examination with 

 a higher power proves that it not only underlies the peripheral 

 part of the archiblast, but that it extends as a thin lamella 

 on the surface of the yolk for a considerable distance towards 

 the zone of the segmentation cavity. Thus, in the preparation 

 which is represented in the present figure 4, I can discern, 

 under a moderately high power (Hartnack, fig. 7), a thin layer 

 of parablast extending on the right side beyond the peripheral 

 thickening of the archiblast for some distance underneath the 

 segmentation cavity J Avhereas on the left side a thin layer of para- 

 blast may be followed on the surface of the yolk, close to the 



