52 GEGENBAUR, ON SAGITTA. 
that of a cell of the mner layers, and those cells whose axes 
thus coincide, had constituted, in the immediately precedent 
stage, a single cell. The two layers are of pretty nearly the 
same thickness, and m each may be seen a long, oval, central 
nucleus. 
But whilst this division is goimg on, the central cavity 
of the vitellus enlarges, the cells by which it is bounded 
gradually becoming more and more remote from the central 
point ; but at the same time its figure is so irregular, that 1% 
would hardly be supposed that it could be the rudiments of 
an important part. But its true nature is soon made mani- 
fest from an opening which, gradually becoming more and 
more evident, places the hitherto closed cavity in communi- 
cation with the exterior. In this way arises a short canal 
perforating the two layers of cells, so that the original cen- 
tral cavity might now be regarded as the cecal termination 
of an invagination or depression commencing from the 
exterior, unless one had not been satisfied, from previous 
observation, that its existence dated from a far earlier period, 
as early even as the first stage of segmentation. 
The essential nature of the process by which the canal is 
formed is, for the most part, unknown; it is an act closely 
connected with the innermost vital phenomena, not only of 
the cells in the immediate region concerned, but which must 
also result from certain changes equally affecting a// the cells 
of which the embryo is composed. 
Observation has shown Gegenbaur that no mere absorption 
of the cells, or, at any rate, that no complete disappearance 
of existing morphological elements takes place, but that the 
opening of the central cavity is the immediate result of a 
separation from one another of certain parts of cells (Zell- 
parthien). 
Were it true that a solution of the cells took place, the 
products of such a process would be visible, and were the 
proceeding one of simple absorption, in some way set up by 
the contiguous cells, the boundary or outline of the canal 
thus produced would have an appearrnce different from that 
‘which it actually presents. The cells forming the boundary 
of the canal are disposed somewhat differently to the rest ; 
simultaneously with the formation of the canal, they have 
the direction of their longitudinal axis changed in such a 
way that this axis in the centre of the embryo no longer 
coincides with that of the other cells which remain un- ° 
changed, but appears rather to be directed towards the canal 
itself. At the same time these cells, both of the internal 
and of the external layer, have hecome somewhat shortened. 
