138 



CCELENTERA. 



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, several (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 cylindrical 

 "head" consisting of nucleus, a minuter middle piece, and a long 

 thread-like vibratile tail (Fig. 41, i). 



(b) 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 amoeboid form, and becomes rich in nutritive material. It also 

 becomes pigmented (Fig. 41, 2). 



Development.'*- The ovum of Hydra is, as we have seen, the success- 

 ful central cell in a nest of interstitial elements which form the rudi- 

 mentary ovary. It is at first amoeboid, 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-cells." 

 With increase of size the ovum changes its form from amoeboid to cake- 

 like, and from that to spherical. Around the spherical ovum a gelatinous 

 sheath is formed. When the limit of growth is reached the nucleus or 

 germinal vesicle divides twice in the usual way, and two polar bodies are 

 extruded at the distal pole. Thereafter, the ectoderm of the parent 

 Hydra yields to the increasing strain put upon it ; and ruptures, allowing 

 the ovum to protrude. By a broad base it still remains, however, 

 attached to the parent, and in this state it is fertilised, the spermatozoon 

 entering by the distal pole (Fig. 41, 4). 



1 As the accounts given by various investigators do not agree, it may 

 be noted that we hav r e here followed that of Brauer (1891). 



