^.2G F. M. BALFOUU. 



form a layer one or two segments deep. The limits of ike 

 blastoderm are not, however, defined by the already com- 

 pleted segments, but outside these new segments continue 

 to be formed around nuclei which appear in the yolk. 

 At this stage there is, therefore, no line of demarcation 

 between the germ and the yolk, but the yolk is being bored 

 into, so to speak, by a continuous process of fresh segmen- 

 tation. 



The further segmentation of the already existing spheres, 

 and the formation of new ones from the yolk below and to 

 the sides continues till the central cells acquire their final 

 size, the peripheral ones being still large and undefined 

 towards the yolk. These also soon reach the final size, and 

 the blastoderm then becomes rounded off towards the yolk 

 and sharply separated from it. 



The Nuclei of the Yolk. 



Intimately connected with the segmentation is the appear- 

 ance and history of a number of nuclei which arise in the 

 yolk surrounding the blastoderm. 



When the horizontal furrows appear which first separate 

 the blastoderm from the yolk, the separation does not occur 

 along the line of passage from the fine to the coarse yolk, but 

 in the former at some distance from this line. 



The blastoderm thus rests upon a mass of finely granular 

 material, from which, however, it is sharply separated. At 

 this time there appear in this finely granular material a num- 

 ber of nuclei of a rather peculiar character. 



They vary immensely in size — from that of an ordinary 

 nucleus to a size greater than the largest blastoderm-cell. 



In PI. XIII, fig. 1, n, is shown their distribution in this 

 finely granidar matter and their variation in size. But what- 

 ever may be their size, they always possess the same charac- 

 teristic structure. This is shown in PI. XIII, figs. 1 and 2, n. 



They are rather irregular in shape, with a tendency when 

 small to be roundish, and are divided by a number of lines 

 into distinct areas, in each of which a nucleolus is to be seen. 

 The lines dividing them into these areas have a tendency (in 

 the smaller specimens) to radiate from the centre, as shown 

 in PI. XIII, fig. 1. 



These nuclei colour red with hsematoxylin and carmine 

 and brown with osmic acid, while the nucleoli or granules 

 contained in the areas also colour very intensely with all the 

 three above-named reagents. 



With such a peculiar structure, in favorable specimens 

 these nuclei are very easily recognised, and their distribution 



