14 PROFESSOR ALLMAN. 



day, from this to the liberation of the young animal embraces 

 a period usually extending over six to eight weeks. 



The first event which occurs during this period is a very 

 remarkable one. After the completion of the shell the 

 boundaries of the cells composing the mass of the germ 

 become indistinct and are finally effaced, the cells themselves 

 have become fused together, and the germ is once more like 

 the unsegmented egg, a single continuous mass of protoplasm. 

 This is filled with pseudo-cells, albumen granules, and, in 

 Hydra viridis, with chlorophyll granules. So enormous a 

 histological retrogradation might lead us to suspect its 

 reality, but Kleinenberg has no doubt of it, and he refers to 

 analogous cases, such as the currents in the protoplasm of 

 Myxomycetes, which show that the cells from which this 

 protoplasm originates have entirely lost their individual dis- 

 tinctness ; Avhile even among the higher animals he can 

 adduce the observations of Bischoff as to the dissolution of 

 the germ-cells in the guinea-pig and the roe-deer. As it is 

 quite certain, however, that this phenomenon does not occur 

 in other hydroids, it can have no general significance for the 

 development of the order. 



In the uniform protoplasm mass thus produced there is 

 next formed a small excentric cavity. This is the foundation 

 of the body cavity. By the solution of the surrounding 

 protoplasm it increases in size and becomes nearly uniformly 

 developed within the germ ; it is filled with a clear liquid. 

 There is thus formed a closed sac — the germ-sac. 



It is clear that the formation of a body cavity by invagina- 

 tion of the Avails, with the significance which Kowalewsky 

 has assigned to it in other animals, does not exist in Hydra, 

 and just as little will it be found in any other hydroid. 



In the condition now described the germ-sac remains for 

 several weeks. In the meantime the outer germ-shell loses 

 its firmness, and is finally burst by the expansion of the 

 contained germ, which now escapes into the surrounding 

 water, covered only by the transparent, elastic, inner shell. 



A further important change next occurs in the germ-sac. 

 The pseudo-cells which had previously been distributed 

 throughout the whole thickness of the walls of the sac have 

 uniformly withdrawn themselves from the surface, so that the 

 sac now presents a clear superficial zone. This is the first 

 indication of the splitting of the walls into the two definitive 

 germ-lamellae. The clear superficial zone is to become the ecto- 

 derm, the darker zone which lies beneath is the endoderm. 



The thickness of the clear outer zone gradually increases, 

 and cells now become differentiated in it. These are the 



