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the outside and develop into cnidoblasts or stinging cells. 

 A nu'mber of them are probably latent ectoderm cells, 

 ultimately becoming such. 



How does Hydra reproduce ? 



In spring and during summer it usually multiplies by 

 gemmation or budding. On the body a pouch-like out- 

 growth appears, and as it lengthens, the free end branches 

 out as a crown of tentacles. This daughter- hydra remains 

 attached for a time and then goes free. 



In autumn the mode of reproduction is sexual. Certain 

 interstitial cells give rise to the testis (one or several swell- 

 ings beneath the tentacles) and the ovary (usually a single 

 swelling near the "foot"); both being produced by one 

 Hydra. 



The ovary contains several potential egg- cells, but only 

 one becomes the large ripe ovum (the macrogamete). 



The ectoderm ruptures and the ovum protrudes. Minute 

 motile sperms (microgametes) are liberated from the 

 testis. When one of these enters an ovum, the result 

 of the conjugation is a fertilised egg- cell or zygote. 



Write a brief Account of the Development of Hydra, 



After fertilisation the egg begins to segment. Segmenta- 

 tion is total (holoblastic) and equal, and the result is a 

 blastula. 



The cells of the wall, by division, give rise to cells which 

 pass into the cavity of the blastula, filling it up ; and 

 thus, by multipolar ingression, the endoderm layer is formed, 

 and there is now a two-layered embryo. 



The ectoderm secretes a protective spiny spherical 

 envelope or shell ; and the enveloped embryo then drops 

 off and remains in the pond mud throughout the winter. 

 After a resting stage the interstitial cells are developed, 

 then the mesogloea ; and finally, when the shell bursts 

 and the elongate embryo emerges, the ccelenteron appears 

 as a space in the endoderm at the upper end, a mouth is 

 formed by splitting, and tentacles are budded out. 



