Dr. W. Salensky on [Héckel’s Gastreea Theory. 7 
ones soon make their appearance. ‘The former afterwards 
form the entoderm, the latter the dermal lamella. Between 
these two foundations of the germ-lamelle a third layer of cells 
is immediately produced, and this forms the middle germ- 
lamella. From this it follows that in Huwazes there is no stage 
which has any resemblance to the Gastrwa, and, indeed, that no 
such stage can occur, because the Gastrula should consist only 
of two layers. But here, even before the conclusion of the 
process of segmentation, all three germ-lamelle are founded. 
In Lumbricus, however, which is systematically very nearly 
allied to Huaxes, a Gastrula-stage occurs. 
In the ontogeny of the Cheetopoda there seems to be no 
Gastrula-stage. From the investigations of Claparéde and 
Mecznikoff* it appears that, after the segmentation of the egg 
in the Cheetopoda, an embryo consisting of two germ-lamelle 
is formed, and that this soon acquires the bands’or tufts of cilia 
&e. and then quits the egg without possessing any stomachal 
cavity. Then the eyes are formed in the embryo, the seg- 
ments of the body become differentiated, and finally the sto- 
machal cavity with mouth and anus is produced. The latter 
consequently takes place long after the time when the embryo 
already possesses its larval organs (Spio fuliginosus, Lumbri- 
conereis sp., and Dasychone lucullana). 
[ need hardly mention that the Ascidia pass through a Glas- 
trula-stage in their ontogeny. ‘This is fully proved by the well- 
known researches of Kowalevsky. 
According to this examination of the stock Vermes, we can 
with certainty detect the Gastrula-stage only in Sagitta, Pho- 
ronis, Lumbricus, the Ascidia, and the Nemertina. In the 
other Vermes the existence of the G'astrula-stage is by no 
means proved by embryological researches. We have seen 
that it does not occur in the ontogeny of most Trematoda, 
and probably of most Nematoda, Bryozoa, Huwaxes, and the 
' Cheetopoda. Perhaps the Leeches might be added to the 
Vermes which pass through a Glastrula-stage (Leuckart, ‘ Die 
menschlichen Parasiten,’ Bd. i. p. 689). 
Before we pass to the other types of animals, we must refer 
to an important phenomenon which is of much significance in 
the correct estimation of the factual evidence. The mode of 
formation of the buccal aperture and of the anterior part of 
the intestinal canal must be mentioned, as in it phenomena 
oceur which might lead to the assumption of a Castrula- 
stage in cases in which really no such stage exists. In the 
ontogeny of alli animals (except the Sponges and some 
* “Beitrage zur Erkenntniss der Entwickelungsgeschichte der Chee- 
topoden,” Zeitschr, fiir wiss, Zool. Bd. xix. pp. 169, 182, and 197. 
