OBSERVATIONS ON DEVELOPMENT OF COMMON TROUT, 127 



yolk, pass into each other, would be no more or less than to 

 admit that the " granules " form the decisive character of 

 the structures in question, and that the "substance of the 

 matrix " plays only a secondary part. The parablast we are 

 dealing with is a living organism ; it grows from an incon- 

 siderably small body into a structure of very great dimen- 

 sions — dimensions which relatively are not very inferior to 

 those of the archiblast ; and that therefore this living struc- 

 ture (parablast), which is almost semifluid and has no limiting 

 membrane, should, while growing on the surface of the 

 likewise soft yolk, which has no limiting membrane either, 

 take up the granules contained in the latter and thus feed 

 itself, is just what we should expect. Under these circum- 

 stances it is clear that the line of boundary between the two 

 structures must be lost, ^. e. obscured by the granules ; for 

 the granules contained in what belongs to yolk pass directly 

 into the granules contained in what belongs to parablast. 

 The fact that the parablast has, at the outset, been forming 

 one unit with what represents the archiblast (blastoderm, 

 Auct.), and while increasing , has spread, i. e. grown over the 

 yolk which underlies the segmentation cavity, is, I think, 

 the most absolute proof that the yolk is as much different 

 from the parablast as it is from the archiblast. 



The nuclei contained in the parablast are different as re- 

 gards shape and size. In the sections of hardened specimens 

 Avhich we are dealing with, only very few appear quite circular ; 

 many are roundish, with their sides more or less flattened on 

 account of being pressed together in small groups ; others 

 are oblong, especially those about the periphery of the archi- 

 blast; and others again are either hour-glass shaped or kidney 

 shaped, or lohecl, furrows and notches extending for various 

 depths into their substance ; that these nuclei, therefore, 

 multiply by division into two or more smaller nuclei is, no 

 doubt, to be inferred from the frequent occurrence of chains 

 of nuclei and of groups of nuclei of different sizes, in which 

 the individual elements appear as sections of one large lobed 

 nucleus. The largest nuclei which deviated least from the 

 circular form were measured in a preparation represented in 

 fig. 6; they were found to be 0*012 mm. in diameter. From 

 nuclei of this size down to the smallest bodies which by their 

 outline and connection with a distinct nucleus could be just 

 recognised as such, all intermediate sizes could be detected. 

 The substance of these nuclei is different from that of the 

 parablast itself: being more transparent, the granules 

 contained in the nuclei irregular and of very different sizes ; 

 besides this, each nucleus possesses a sharp outline as if 



