94 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



luit it one reaches an ovary containing a ripe macro- 



;his and syngamy takes place. 



fertilized egg now proceeds to undergo the process of 



segmentation consisting of fission repeated over and over again. This 



:n tin- formation of a blastula a mass of cells forming a sphere 



1 in a sinule layer round a central cavity. As development 



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



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



solid and consists of two distinct layers of cells those 



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



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



.md the endoderm. 



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



i lv the ectoderm and differing in appearance in different species 



ol 1 1 \-dr a. It drops off the parent and remains in the mud at the bottom 



.if the water tor it may be a prolonged period until conditions again become 



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



e ferment to soften the shell, makes its way out and gradually 



pa into a typical small Hydra. 



HYDROMEDUSAE 



nst ruet ive to compare with Hydra those animals grouped together 



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



implicated than it is in Hydra. It is also a characteristic feature 



ip that while asexual reproduction by budding takes place 



!i\iduals so arising do not as a rule become separate but remain 



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



OB ELI A 



A good illustrative example of the Hydromedusae is the common 



' : 'lia. To the naked eye a colony of Obelia looks like a 



Ahitish thread creeping over the surface of a seaweed or stone or 



snr11 ;r "ft at intervals little branches which project freely into 



of these branches can be seen with a magnifying lens 



to be ben; in a characteristic /ig/a- manner and to give off from the outer 



1 An embryo i- .1 v<,un- developing individual, which is contained within the body 

 s " hln > l"<>t'-etr other envelope. A larva is on the other 



'pirn: individual, diifi-rinj,' in form from tin- adult, but not con- 

 Utard within th.. i,,,dv ,,f the parent or other protective envelope, 



