Dr. W. Salensky on Hiéickel’s Gastreea Theory. 17 
many instances occur at a much later period, after the com- 
pletion of the segmentation. In most such cases the segmen- 
tation-cells are regularly evolved ; there are 2, 4, 8, 16, &c. 
cells, which further divide with the same regularity; in a word, 
a regular segmentation takes place, producing a solid sphere 
consisting of uniform cells. For this stage we may retain the 
name of “ Morula,’ by which Hiickel indicates the so-called 
mulberry-stage of segmentation. The Morula may become 
differentiated in different ways. It may at once form the 
embryo itself, becoming covered with a cuticular membrane 
and cilia, and escaping as a larva—as is the case, for example, 
in the digeneous Trematoda* (Amphistomum subclavatum &c.), 
according to the observations of K. van Beneden. ‘The larva 
of these animals consists of uniform cells and is covered ex- 
ternally with a ciliated membrane. The larva of the Trematoda 
can probably become further differentiated and even acquire a 
stomachal cavity. 
The Cestoda pass through the Morula-stage in the egg. 
Before the embryo escapes from the egg, the differentiation of 
its cells commences in it. This differentiation differs in its 
results from the differentiation of the germ-lamelle, although 
the processes are the same in both cases. In consequence of 
the differentiation there is produced a body consisting of two 
layers (a central and a peripheral one). But these two layers 
pass through their further evolution in a somewhat different 
manner than in the analogous processes of differentiation in 
other animals. In the Cestoda the peripheral layer becomes 
converted into a ciliary envelope (or its homologue), and the 
central layer into a six-hooked embryo. rom the researches 
of EK. van Beneden we obtain the data for a comparison of the 
developmental history of the Zante with that of the Bothrio- 
cephalt. This naturalist has shown that after the egg of the 
Cestoda (both Tanie and Bothriocephali) has passed through 
a Morula-stage (mulberry form), it becomes differentiated into 
two layers, peripheral and central t. (Similar processes had 
been previously observed in the Bothriocephalide by Kdélliker, 
Mecznikoff, and Knoch.) These two layers are then developed 
in different ways: from the outer one is formed, in the Bo- 
thriocephalide, the embryonal envelope (in the Zeni@ it en- 
* Properly speaking a differentiation has already taken place here, in- 
asmuch as the peripheral cells have the cilia, which the central ones do 
not possess. But this differentiation is essentially distinct from that of 
the Cestoda and other animals, and it does not lead to the formation of 
the germ-lamelle. 
+ Recherches &c.,in Mémoires couronnés de l’Acad. Royale de Belgique, 
tome xxvi. 
Ann. & Mag. N. Hist. Ser. 4. Vol. xv. 2 
