EEPOET OX THE HYDEOIDA. 
xlvii 
After these changes in the prismatic cells a continuous membrane can be seen extend- 
ing over the entire surface of the germ. By maceration it can be separated from the 
subjacent plasma, and by its resistance to the action of chemical reagents it appears to be 
of a chitinous nature. The first formed membrane increases in thickness by the formation 
below it of many successive layers, and the germ instead of being surrounded by a layer 
of naked prismatic cells has become enclosed in a thick chitinous shell. 
After this outer shell has been completed there is formed between it and the germ a 
second envelope. This is a thin, structureless, transparent, and very elastic pellicle. It is 
probably formed by the hardening of an excreted liquid. 
Kleinenberg regards the outer shell as formed by a total transformation of the entire 
outer cell stratum. The shell is accordingly an epidermal structure, and the first 
differentiation of the germ of Hydra thus consists in the formation of a peripheral one- 
layered cellular lamina, the component cells of which die, and have their plasma trans- 
formed into chitin so as to form a firm shell which protects the remaining part of 
the germ from destruction during the long period of its subsequent development. 
The first organ which proceeds from the Hydra germ is thus a transitory one which takes 
no direct part in the development, and which on the liberation of the definitive body is 
simply cast off. 
As soon as the shell is completed the union of the germ with the maternal body is 
dissolved, and the germ falls off and sinks to the bottom. A remarkable change now 
takes place in the structure of the germ. This consists in the fusion of all its cells into 
an undifferentiated plasmodium. All trace of a cellular structure disappears, and the 
germ becomes again, like the unsegmented egg, a single large plasma mass, thickly filled 
with albumen granules, pseudo-cells, and chlorophyll grains. 
In this uniform mass there is now formed a small cavity which lies excentrically near 
the surface. This is the foundation of the body cavity. It enlarges in all directions, 
and it is clear that it arises by a true liquefaction of a great part of the substance of the 
germ. The walls, however, of the hollow germ thus formed are as yet uniform and show 
no trace of differentiation. 
In this state the germ remains for many weeks, during which the outer shell becomes 
softer, and is finally burst and cast off. The germ, however, which it had enclosed con- 
tinues to be overlaid by the elastic transparent inner shell which lies close upon its surface. 
In the hitherto uniform walls of the germ we may now distinguish two layers. 
This condition is caused by the retreat of the pseudo-cells from the more superficial parts 
of the wall into its deeper parts, resulting in the appearance of an external clear layer 
and an internal darker one. This is the first indication of the differentiation of the two 
germinal layers. Out of the clear layer the ectoderm is formed, out of the darker one 
the endoderm. By a further differentiation these layers become changed from a continuous 
plasma into cellular laminae. 
