128 HYDRO IDE A. 



cavity is formed, and provisional- tentacles arise as a series of conical 

 involutions which subsequently become evoluted. Permanent tentacles are 

 formed as conical papilla? on a truncated oral process. After hatching it 

 has a few days' free existence, and then becomes attached, and loses its 

 provisional tentacle?. 



Although Hydra itself constitutes the simplest type of Hydrozoon, its 

 development, which has been fully investigated by Kleinenberg (No. 161), is 

 in some respects a little exceptional. The segmentation is regular, but a seg- 

 mentation cavity is not formed. The peripheral layer of cells gradually 

 becomes converted into a chitinous membrane, which is perhaps homologous 

 with the perisarc of marine forms. Between the membrane and the germ a 

 second pellicle makes its appearance. The above changes require about 

 four days for their completion, but there next sets in a period of relative 

 quiescence which lasts for some 6 8 weeks. During this period the 

 remaining development is completed. The cells of the germ first fuse 

 together. In the interior of the protoplasm a clear excentric space arises, 

 which gradually extends itself and forms the rudiment of the gastric cavity. 

 The outer shell in the meantime becomes less firm, and is finally burst and 

 thrown off, owing to the expansion of the embryo within. 



The outermost layer of the protoplasm becomes, relatively to the inner 

 layer, clear and transparent, and there thus arises an indication of a division 

 of the walls of the archenteric cavity into two zones, or layers. These layers, 

 which form the epiblast and hypoblast, are definitely established on the 

 appearance of cells with contractile tails 1 in the clear outer zone, between 

 which the interstitial epiblast cells subsequently arise. 



The embryo, still forming a closed double-walled sack, elongates itself, 

 and at one pole its wall becomes very thin. And at this point a rupture 

 takes place which gives rise to the mouth. Simultaneously with the mouth 

 the tentacles become formed as hollow processes, according to Mereschkowsky 

 two being formed first and subsequently the others in pairs. Very shortly 

 afterwards the hitherto uniform hypoblast becomes divided up into distinct 

 cells. The thin inner pellicle which persists after the rupture of the outer 

 membrane becomes in the meantime absorbed. With these changes the 

 embryo practically acquires the characters of the adult. 



Trachymedusse. Amongst the Trachymedusae, which as has now 

 been satisfactorily established develop directly without alternations 

 of generations, the embryology of species both of the Geryonidae 

 and the ^EginidaB has been studied. 



In all the types so far investigated the hypoblast is formed by 

 delamination, and there is a more or less well-marked planula 

 stage. 



The development of Geryonia (Carmarina) hastata has been 

 studied by Fol (No. 155) and Metschnikoff (No. 163)". The ovum, when 

 laid, is invested by a delicate vitelline membrane and mucous covering. 

 Its protoplasm is formed of an outer granular and dense layer, and a 



1 These cells are the so-called nerve-muscle cells. Their nature is discussed in the 

 second part of this work. 



2 In the succeeding account I have followed Fol, who differs in some minor points 

 from Metschnikoff. 



