94 ZOOLOGY FOR MEDICAL STUDENTS chap. 



of male gametes — but if one reaches an ovary containing a ripe macro- 

 gamete, it penetrates this and syngamy takes place. 



The zygote or fertilized egg now proceeds to undergo the process of 

 segmentation — consisting of fission repeated over and over again. This 

 results in the formation of a blastula — a mass of cells forming a sphere 

 and arranged in a single layer round a central cavity. As development 

 proceeds cells derived from the cells of the wall drop into the cavity 

 and eventually fill it. The mass of cells, or the embryo i as we now call 

 it, is at this stage solid and consists of two distinct layers of cells — those 

 which formed the wall of the blastula and those which fill its cavity. 

 These are the two primary cell-layers of the individual — the ectoderm 

 and the endoderm. 



The embryo now comes to be enclosed in a protective chitinous shell, 

 secreted by the ectoderm and differing in appearance in different species 

 of Hydra. It drops off the parent and remains in the mud at the bottom 

 of the water for it may be a prolonged period until conditions again become 

 favourable. When this happens the embryo, apparently by the secretion 

 of a digestive ferment to soften the shell, makes its way out and gradually 

 develops into a typical small Hydra. 



Hydromedusae 



It is instructive to compare with Hydra those animals grouped together 

 under the name HYDROMEDtrsAE, in which the life-history is somewhat 

 more complicated than it is in Hydra. It is also a characteristic feature 

 of this group that 'while asexual reproduction by budding takes place 

 the individuals so arising do not as a rule become separate but remain 

 throughout life connected together in the form of a community or colony. 



Obelia 



A good illustrative example of the Hydromedusae is the common 

 marine genus Obelia. To the naked eye a colony of Obelia looks like a 

 bit of whitish thread creeping over the surface of a seaweed or stone or 

 shell and giving off at intervals little branches which project freely into 

 the water. Each of these branches can be seen with a magnifying lens 

 to be bent in a characteristic zigzag manner and to give off from the outer 



1 An embryo is a young developing individual, which is contained within the body 

 of the parent or within a protective shell or other envelope. A larva is on the other 

 hand a young developing individual, differing in form from the adult, but not con- 

 tained within the body of the parent or other protective envelope. 



