ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 515 



After an account of the mode of cleavage, we find that when the 

 internal cavity of the embryo is completely filled up the earlier 

 blastomeres take on the appearance of true cells, and divide most 

 actively in the outer layer of the epiblast. In H. fusca the egg is 

 closely attached to the parent, while in H. aurantiaca the ectodermal 

 cells of the parent which are in relation to the egg, gradually become 

 glandular in character, and give rise to a special organ which produces 

 a kind of mucous substance, by means of which the egg is attached, 

 and from which a special layer is developed around it. The epiblast 

 and hypoblast become separated, and the surface of the former 

 becomes warty, while its cells develope pseudopodioid lobate pro- 

 cesses ; and there are indications of a chitinous egg-shell. Each 

 epithelial cell then forms a cylindrical body, at the base of which are 

 some yolk-spheres. The egg now separates from the body of the 

 parent, and becomes attached to various objects. The glandular cells 

 of the disk, now developed, excrete a glairy mucous substance, after 

 the secretion of which the special cells lose their peculiar characters. 



About this period there is a histolysis of the hypoblast, the cells 

 of which lose their distinctness, the plasma becoming concentrated 

 around the nucleus ; the epiblast and hypoblast are not now so sharply 

 distinguished. The cells of the former undergo a retrograde meta- 

 morphosis ; and we find that the primary epiblast goes to form the 

 egg-shell, while the yolk-membrane and the mucous layers are com- 

 pletely used up, and take no part in the formation of the secondary 

 epiblast. 



The author points out that the doctrine that if the epiblast is cast 

 off the nervous layer ought to be most external (and in fact the ordinary 

 ectoderm of Hydra consists of an uninterrupted layer of nerve-cells), 

 is supported by his observations. He regards the histolysis (meta- 

 morphosis) of Hydra as a direct result of external influences, which 

 act in quite a special way. In the lower animals, and especially in 

 the Coelenterata, we cannot say that the germinal layers play so 

 definite a part as in the higher forms. These exceptions and varia- 

 tions from what are regarded as general laws, which we see in the 

 lower forms, are due to adaptations, the capacity for which in these 

 organisms is much more considerable than in the higher animals; 

 and thus we may understand how it is that the outer layer of the 

 morula forms a shell. 



Alternation of Generations in Hydro-mednsaB.* — Mr. W. K. 



Brooks thinks it is hardly possible that the form of development 

 which we now find in most of the Hydro-medusa can bear any close 

 resemblance to their primitive life-history ; and there are many 

 reasons for believing that alternation of generations has gradually 

 arisen through the modification of " metamorphosis." 



In Cunina we seem to have the ancestral form of development, 

 a direct metamorphosis without alternation. The interesting and 

 remarkable life-history of Cunina was first described by Professor 

 M'Crady, who found inside the bell of a hydro-medusa, Turritopsis, a 



* Jolms Hopkins University Circulars, ii. (1883) p. 73. 



2 L 2 



