Dr. G. Biitschli on the Gastrea-Theory. 377 
We therefore take such a two-layered plate as the starting- 
point, and will see, in the first place, how a gastrula-like form 
could be developed from it. Clearly by the simple incurvation 
of such a placula towards the entodermal surface, and final 
conglobation until the formation of a blastopore. But, one 
naturally asks, what advantage will such an incurvation of 
the plate possess ? Upon this point various statements may 
be made. The incurvation of the plate, and particularly of 
its entodermal layer, which is at first but slight, will enable 
large nutritive masses to be attacked simultaneously by a 
number of neighbouring cells, and at the same time the cavity 
of the underside will serve as a sort of trap in which prey 
may be captured and held fast, if the curved plate lowers 
itself over a prey resting upon some support. Both advan- 
tages will become more and more marked the more the curva- 
ture makes itself felt; and the disadvantage which consists 
in the fact that the invagination-aperture gradually diminishes 
in size, may be compensated by the increased security of the 
prey when captured, of which it is the cause. 
The next question, however, is whether a course of deve- 
lopment such as we have constructed hypothetically is any- 
where ontogenetically represented, and this is actually the case. 
It occurs mostly in certain Nematoda, such as Cucullanus 
according to Butschh*, and in the main also in Rhabdonema, 
according to Géttet. In these cases the result of the process 
of segmentation is a two-layered cell-plate, a true placula 
consisting of ectoderm and entoderm, which afterwards be- 
comes incurved as above described, and passes into the 
gastrula-stage. Indications of the same course of develop- 
ment are also frequently to be seen, although usually the 
plate-form does not appear in such purity, seeing that between 
the two layers there occurs a small accumulation of fluid, 7. e. 
a segmentation-cavity has been developed, which was entirely 
wanting in the first-mentioned cases. I will here cite a few 
examples which distinctly show such a plate-form :—Lumbri- 
cus according to Kowalevsky, Paludina, Chiton, according to 
Kowalevsky’s investigations ; further, Sag¢tta, in which the 
segmentation-cavity is very slightly developed; and, finally, 
colonies make me regard the first-mentioned view as the more probable. 
Unfortunately, however, I am unable to cite any plausible advantages for 
the commencement of the bilamellarity of the plate, although this may 
perhaps have occurred simply by special conditions of growth, and at any 
*rate one cannot see that any disadvantage could accompany the com- 
mencement of bilamellarity. 
* * Zeitschr. f. wiss. Zool. Band xxvi. p. 103. 
+ Abhandlungen zur Entwicklungsgeschichte der Thiere, Leipzig, 1882, 
Ann. & Mag. N. Hist. Ser. 5, Vol. xiii. 25 
