FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 215 



arched, commences to flatten towards the centre. As a result of this flattening, 

 a vertical furrow is slowly pushed down towards the surface of the yolk, as 

 seen in optical section, but stops short some distance above the latter. A study 

 of stained sections of this stage shows several points which cannot be made 

 out in the living egg. The upper portion of the germinal area contains very 

 little food yolk, but between this and the main body of the yolk material there 

 is a somewhat triangular area containing a larger quantity of yolk imbedded in 

 the protoplasm, and from the base of which the protoplasmic processes are 

 pushed down into the yolk. This area is actively engaged in assimilating food 

 material, and is crowded with particles of yolk. At the time, therefore, that 

 the upper and older portion of the germinal area is ready for segmentation, the 

 material included in the lower portion adjoining the yolk is not so far advanced. 

 It is one of the recognised laws of segmentation that the rapidity with which 

 any part of an ovum segments varies, ceteris 'paribus, with the relative amount 

 of protoplasm it contains. The protoplasm in this vegetative portion of the 

 germinal area contains as yet too much undigested yolk for it to take part in 

 the segmentation process. Thus the first furrow progresses more slowly as it 

 approaches this area, and for the time being is arrested before it has reached 

 the base of the germinal disc. Another point is also shown in sections of this 

 stage, which will be considered more fully at a later period, but which should 

 be mentioned here. The first furrow is not a continuous plane of cleavage in 

 the first instance. There are in the line of cleavage a series of small vacuoles, 

 which, as they become more elongated, run together, and so form a considerable 

 portion of the furrow. Thus the first two segmentation spheres are in part 

 separated by a process of vacuolation. The first two segmentation spheres 

 become entirely separated from one another at their upper poles, the series of 

 vacuoles aiding considerably in this process. The germinal area is thus divided 

 into two portions, which are completely separated above, but which are united 

 at the base, the first furrow not having completely penetrated to the base. 



After the first furrow has been formed, a comparatively quiescent stage 

 follows, during which a part of the germinal protoplasm again collects in a 

 mound at the yolk pole. This is not an accidental occurrence, but has been 

 already observed by Kupffer, and I have had frequent opportunities of 

 observing this process. During this quiescent period it is usual for a nucleus 

 to make its appearance towards the centre of each segmentation sphere in 

 pelagic fish ova. Such nuclei in the living ovum have the appearance of a 

 more transparent area, usually distinctly marked off from the surrounding 

 protoplasm. They usually disappear again before the next active stage begins, 

 again to reappear during the following quiescent period. I have not observed 

 such nuclei in the living egg of the herring at this stage, nor have I been 

 able to make them out in stained preparations. 



VOL. XXXIII. PART I. 2 G 



