'34 



CCELENTERA. 



The middle lamina, representing the mesogloea, is a thin homogeneous 

 plate, on each side of which lie the muscular roots of ectodermic and 

 endodermic cells (Fig. 57, D). 



It is historically interesting to notice the important step which was 

 made when, in 1849, Huxley definitely compared the outer and inner 

 layers of the Ccelentera with the epiblast and hypoblast which embryo- 

 logists were beginning to demonstrate in the development of higher 

 animals. Not long afterwards, Allman applied to the two layers of 

 Hydroids the terms ectoderm and endoderm. 



The division of labour among the cells of Hydra is not very strict, 

 but already the essential characteristics of ectoderm and endoderm are 

 evident. We may summarise these as follows, comparing them with 

 the characteristics of epiblast and hypoblast in higher animals : — 



The reproductive organs.— (a) From nests of repeatedly-dividing 

 interstitial cells, seveial (1-20) simple male organs or testes are formed. 

 Each consists merely of a clump of male elements or spermatozoa, 

 bounded by the distended ectoderm. Through this the spermatozoa 

 are extruded at intervals, and one may fertilise the ovum of the Hydra. 

 In other words, self-fertilisation, which is very rare among animals, 

 may occur. The spermatozoon is a motile cell, with a minute cylin- 

 drical "head " consisting of nucleus, a more minute middle piece, and a 

 long thread-like vibratile tail (Fig. 58, 1). 



(d) Usually there is but one female organ or ovary, but in H. fusca 

 as many as eight have sometimes been observed. The ovary arises like 

 the testes from a nest of interstitial cells, one of which becomes the 

 ovum. In rare cases there are two ova. The ovum is at first amoeboid 

 and transparent, but, like many other ova, it feeds on its neighbours, 

 loses its amceboid form, and becomes rich ,in nutritive material and 

 in pigment. The same process of exploitation is well seen in the 

 oogenesis of Tubularia larynx, a common marine polype. It illus- 

 trates the struggle for existence among germ cells. 



Development.— The ovum of Hydra is the successful central 

 cell in the ovary. It is at first amceboid, and becomes more and 

 more rich at the expense of its neighbours. Their remains (perhaps 

 nuclei) accumulate within the ovum as "yolk spherules" or " pseudo- 



