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382 Dr. G. Biitschli on the Gastreea- Theory. 
plan, such as we might expect to meet with upon the above- 
stated hypothesis. 
As I had formulated the outlines of the hypothesis before 
the publication of Schulze’s discovery, I was surprised to find 
in the latter, in a certain degree, such unexpected confirma- 
tion of purely speculative considerations. Although, there- 
fore, the present hypothesis is not based upon the interesting 
Trichoplax, nevertheless the latter has furnished the induce- 
ment to publish the speculation. Jam, indeed, not convinced 
that our science will derive any direct gain from the pursuit of 
such speculative endeavours, but perhaps they may furnish 
some incitement to a more accurate investigation of the onto- 
genetic history of the formation of the blastule and gastrule, 
by means of which the most probable hypothesis, 7¢.e. the 
one which is at once freest from contradictions and most ex- 
planatory, will finally obtain the victory. 
EXPLANATION OF PLATE XV. 
On the accompanying Plate I have endeavoured, by means of a few 
diagrams, to elucidate the ideas developed in the preceding pages as to 
the reference of tle different forms of blastulee and gastrule to a common 
starting-point. That process which I regard as the most primitive is 
represented in figs. la-2a-d. Fig. 1, binary division; fig. 2a, eight- 
celled stage: by an equatoreal segmentation the (tinted) entodermal 
elements haye separated from the (white) ectodermal elements. Of 
course, under certain circumstances this separation may take place either 
earlier or later. Fig. 2b, section. The ectodermal and entodermal cells 
have increased and now form a very distinct placula. In fig, 2 this 
commences its curyature towards the entodermal surface; and this leads, 
finally, as in fig. 2d, to complete invagination of the entoderm, 
Figs. 1 a, 2a, 3a-8c show the development of the so-called invagina- 
tion-blastula. The further development of the stage fig. 2a 
is altered in this way: the severed ecto- and entodermal cells 
are separated from each other by the development of a seg- 
mentation-cayity (fig. 3a), and finally lead to the blastula. 
This, of course, consists of two different parts—an ectodermal 
and an entodermal section. Itis particularly to be noted, how- 
ever, that the relative size of these two sections appears to be 
very different in the different invagination-blastule, which 
may be referred to an earlier or later severance of the ecto- 
and entodermal elements, as well as to the relative quantities of 
these two elements. The stage fig. 3d passes finally, in the 
well-known manner, by invagination (fig. 3c) into the gastrula- 
stage. 
Figs. 1a-1 e represent the delamination-blastula. Fig. 1 a, binary divi- 
sion. Fig. 10, eight-celled stage, but with only four cells drawn 
in side view. ven in the four-celled stage the tendency to- 
wards incurvation will be expressed, leading to a change of 
position in the cells, so that these now direct their entodermal 
parts axially. At the same time it may happen that the groove 
which effects the transition from the four- to the eight-celled 
