OBSERVATIONS ON DEVELOPMENT OF COMMON TROUT. 125 



dimensions. First of all it is found to possess a considerable 

 thickness, not only around the peripheral thickening of the 

 archiblast, but also at the peripheral part of the floor of the 

 segmentation cavity ; and, secondly, with a high power the 

 parablast may be traced to extend for a considerable distance 

 towards the centre on the surface of the yolk as a thin granu- 

 lar lamella, including nuclei, and possessing here and there 

 swellings, which appear to be wedged in between the large oil 

 globules of this part of the yolk. There is little to be added 

 at present with regard to the nuclei of the parablast, except 

 that in the part surrounding the edge of the archiblast they 

 (nuclei) are elongated and form extensive rows. 



A section taken at a right angle through the peripheral 

 part of the specimen represented in fig. 5 naturally shows 

 the segmentation cavity included between the archiblast and 

 the thickened parablast, the lower surface of the former 

 showing the loosely connected elements of various sizes, and 

 the latter being conspicuous by its uniformly granular sub- 

 stance and the larger or smaller groups of its nuclei. 



It may be mentioned already here, that amongst the ele- 

 ments lying loosely on the floor of the segmentation cleft 

 (see above), there are seen here and there apparently free 

 nuclei, some of considerable dimensions, and resembling 

 nuclei contained in the parablast. 



At a somewhat later stage (fourteen days) the thin part of 

 the archiblast consists only of two layers of cells, very dis- 

 tinctly separated from each other (see fig. 6), the upper layer 

 (corneous layer) being continuous with a similar layer at the 

 thickened periphery of the archiblast, and the lower Avith what 

 corresponds to the nervous or second layer (see my first com- 

 munication on this subject). In the peripheral thickening 

 (of course, including that portion of it which becomes trans- 

 formed into the embryo) there are present underneath the 

 two strata just mentioned several layers of cells, which, 

 as has been stated already, form a less continuous mass 

 than the superficial cells. The parablast is found to have 

 increased considerably, not only in thickness, but also in 

 breadth, as it may now be traced very distinctly over almost 

 the whole surface of the yolk, i. e. almost along the whole 

 extent of the floor of the segmentation cleft. As in the 

 preceding stage so also in this, the greatest thickness of the 

 parablast is no more to be found externally to or directly 

 underneath the edge of the archiblast, but internally to it, 

 i. e. on the peripheral portion of the floor of the segmentation 

 cleft. Towards the centre the parablast becomes thinner, 

 and in some places it is just possible to trace it from one 



