14 Dr. W. Salensky on Héckel’s Gastreea Theory. 
sarcode-like body-mass, into which various small organisms 
find their way as nourishment and are there digested in the 
same way as in the Infusoria. They have consequently a 
mouth and the intestinal foundation (Darmanlage), but are des- 
titute of the stomachal cavity. We have no grounds for ex- 
plaining the absence of the intestinal cavity in these animals 
as a consequence of retrogressive metamorphosis *. 
These two circumstances (namely, 1, the diversity of organi- 
zation of the embryos of different animals at the time of the 
formation of the intestinal cavity, and, 2, the accelous condition 
of some Turbellaria) show quite sufficiently that we are not 
in a position to derive the embryonal processes from the Gas- 
trula, nor consequently to accept the Gastrea as the stock- 
form for the phylogenetic development of the Metazoa. They 
show that animals may possess the intestinal foundations, 
without arriving at the formation of the intestinal and stoma- 
chal cavities. From this it follows in general that we hardly 
have any reason for assuming the presence of the stomachal 
cavity in the stock-form of all Metazoa. 
This applies also to the two primary germ-lamelle, which 
constitute the second important character of the Gastrula-stage. 
Is the middle germ-lamella only developed when the two pri- 
mary germ-lamelle, the exoderm and entoderm, are already 
at least perfectly formed, even if they do not together consti- 
tute a Gastrula-form? By no means. We can only say that 
the middle lamella originates somewhat later than the other two 
germ-lamelle ; but in the majority of cases it originates long 
before the stomachal cavity is formed, and it may even originate 
at a time when the process of segmentation is not quite com- 
pleted. After this differentiation of the first segmentation- 
cells, the segmentation may still go on in all these layers of 
cells. We know of such cases with the greatest certainty, from 
investigations which have been carried on with perfect accu- 
racy. QOnesuch instance we know in Huaxes, from the inves- 
*TIt might be objected that retrogressive metamorphosis is by no means 
always dependent on parasitism, but that there are animals which pass a 
free existence and yet undergo a retrogressive metamorphosis, e. g. the 
males of the Rotatoria. But what is usually regarded as the retrogressive 
metamorphosis of the male Rotatoria is really only an arrest of develop- 
ment, and consists in the development of these animals remaining sta- 
tionary at a certain stage, namely at that stage in which they possess no 
intestinal cavity, but only the foundation for the intestine. In the females 
a cavity, the intestinal cavity, is formed in this foundation, but not in the 
males. This mode of development presents essential differences from re- 
trogressive metamorphosis, as in the latter the animals first show a higher 
organization and afterwards lose it. (See my “ Beitrage zur Entwickelung 
des Brachionus urceolaris,” in Zeitschr. fiir wiss. Zool. Bd. xxii.) 
