10 PROFESSOR ALLMAN. 



At the same time, peculiar structures make their appear- 

 ance in the interior of the egg. In Hydra viridis these 

 bodies present the appearance of sharply-coloured spherical cor- 

 puscles, which lie embedded in the protoplasm. When liberated 

 and treated with diluted acetic acid, they are seen to possess 

 a thick wall surrounding a cavity ; the wall is flattened at 

 one spot, nnd to the inner side of this is attached the base of 

 a thick conical body, Avhich projects deeply into the cavity. 



It is these bodies which, according to Kleinenberg, have 

 been mistaken by Ecker for embryonal cells, resulting from 

 the segmentation of the e^^. Kleinenberg, however, cannot 

 regard them as cells ; they take no direct part in the building 

 of the embryo, and remain as intercellular form-constituents, 

 which have manifestly the signification of material reserved 

 for future use -, they become gradually broken up. He names 

 them " pseudo-cells," and compares them to the so-called 

 yolk-spherules of the eggs of vertebrata. The resemblance 

 of these pseudo-cells of H. viridis to certain bodies which I 

 have elsewhere (' Gymnoblastic Hydroids,' p. 59) described 

 as occurring free along with the eggs in the gonophore of 

 Antennularia is very striking. 



The formation of pseudo-cells continues until these bodies 

 constitute a large proportion of the mass of the ovum. The 

 egg has now attained in its greatest diameter the dimension 

 of about 1*5 mm. ; the projections of its edges are gradually 

 withdrawn and become fused into the central mass, and the 

 egg has assumed the form of a smooth, ovoid body. 



About the time when the formation of the pseudo-cells is 

 finished a retrograde metamorphosis begins in the germinal 

 vesicle and spot which finally results in their disappearance. 

 It is here of importance to notice that the germinal vesicle 

 has disappeared without a trace long before the occurrence of 

 fecundation. The egg has now replaced nearly the whole of 

 the original ovarium, and is closely invested by the outermost 

 layer of the ectoderm. 



From the account now given it will be seen that the egg is 

 developed out of a single cell of the ovarium, that it main- 

 tains to the last its morphological value as a simple cell, dif- 

 fering from the other cells of the ovarium by its independent 

 growth and development — ^an important fact in its relation to 

 the essential nature of the egg, and in its bearing on the 

 general question as to Avhether the completely formed animal 

 egg is a simple cell or a compound structure. 



Soon after the disappearance of the germinal vesicle the 

 time arrives when the egg is to escape from its confinement. 

 An opening is formed in the summit of the membrane within 



