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AN INTRODUCTION TO ZOOLOGY 



is effected in the normal manner by the penetration of a single 

 spermatozoon, and the fusion of the two pronuclei to form a zygote 

 with its segmentation nucleus. 



The process of cleavage or segmentation now sets in, and 



FIG. 52. Development of Hydra, adapted from Brauer, 



A., the mature ovum full of yolk granules and attached to the parent ; B., section through the 

 blastula, some of whose cells are already beginning to pass inwards ; C., section through gastrula 

 showing the irregular entoderm cells commencing to fill the blastocoel ; D., the embryo within 

 protective sheath flattening out and the entoderm cells arranged to form a layer around the enteron; 

 E., embryo escaping from sheath ; D. and E., less highly magnified than remainder. 



bl., blastocoel ; ec., ectoderm ; en., entoderm. 



it is during this period in H. viridis that the ovum becomes infected 

 with the green algal cells. Two meridional divisions cut the original 

 cell into four, and a subsequent equatorial split into eight, cells 



