OBSERVATIONS ON DEVELOPMENt OF COMMON TROUT. 129 



peripheral thickening of the archiblast and the parablast, it 

 appears probable that the deepest cells of the peripheral thick- 

 ening of the archiblast owe their origin to that part of the para- 

 blast. In examining preparations like those represented in 

 figs. 5, 6, 7, and 8, it may be easily ascertained that many of 

 the cells lying on the floor of the segmentation cavity, especially 

 many of those forming a loosely connected deep stratum of the 

 peripheral thickening of the archiblast, are formed out of the 

 parablast. Looking, for instance, at a preparation represented 

 in figs. 7 and 8, it requires no difficulty, I think, to find not 

 only nuclei perfectly identical with those of the parablast, but 

 also larger masses, which in all characters denote their origin 

 from the parablast. In fig. 8, especially, these facts come 

 out with certainty; that huge element represented in the 

 figure as attached to the lower layer of cells of the archiblast 

 cannot conceal its place of birth; and the same may be found 

 on close inspection with many large multinuclear elements, 

 not only on the floor of the segmentation cavity, but also on 

 the lower surface of the archiblast. It thus follows that 

 cells of the deeper layer of the archiblast — in- the peripheral 

 thickening (including, of course, the embryonal shield), as 

 well as in the central thinner parts — are derived, directly or 

 indirectly, from the parablast. The assertions, therefore, of 

 Rienek,^ myself,'' and Strieker (Strieker's * Histology,' see the 

 chapter on development of simple tissues), that the peripheral 

 thickening of the archiblast owes its origin, to a great extent, 

 to a displacement of elements at first situated on the floor of 

 the segmentation cavity, but originally derived from the 

 archiblast ; and likewise the assertion of Oellacher," that all 

 the elements of the peripheral thickening owe their origin to 

 the continuous reproduction of its cells, i. e. those contained in 

 the peripheral thickening; and, finally, the assertion of Gotte^^ 

 and Haeckel,^^ to the effect that the deeper layers of the 

 archiblast owe their origin entirely to an ingrowth of the 

 edge of the archiblast (Gotte, 1. c, p. 684 : " Der Rand 

 des Keimes schlagt sich auf einer Seite um und breitet sich 

 an der unteren Flache des Kenines aus ") ; all these asser- 

 tions are but partially correct. I cannot express myself 

 with anything like certainty on this supposed ingrowth of 

 the edge of the archiblast as maintained by Gotte, and to 

 some extent accepted also by Balfour; but this much I have 

 seen, that the appearances presented by the peripheral 

 thickening in general, and the embryonal shield especial, do 

 not strictly necessitate the assumption of an ingrowth of the 

 archiblast. On the other hand it is certain, as we have 

 shown, that the deeper layers of cells of the archiblast are in 



\0L. XVI. NBW SER. I 



