38 



ESSENTIALS OF ZOOLOGY 



At this stage, then, the embryo consists of an ectoderm and 

 endodermal cells protected by a double envelope. The outer 

 layer of the case may be roughened by spiny processes. In 

 this condition the embryo may remain many months. In 

 temperate regions the eggs are formed on the approach of 

 winter, and thus the protected embryos are carried over the 

 cold months of the year. They lie in the mud during the 

 winter in a dormant state, and are called to further develop- 

 ment with the coming of warmer conditions in the spring. 



Ovary and ovum 



Embryo 



Planvla Young stages 



FIG. 16. The reproductive cells of Hydra and stages in development. 



During this period, however, Hydra is liable to be widely 

 dispersed by river currents, and the egg cases may be desiccated 

 and carried by winds. 



Before the winter rest takes place the embryo usually is a 

 two-layered one, the endoderm having formed itself into a 

 layer .on the inner face of the ectoderm. A mesogloea is 

 secreted between the two, and interstitial cells are formed. 

 In the spring, and in tropical conditions on the coming of the 

 wet season, the embryo escapes from the shell as a two-layered 

 larva or planula and creeps forward by an end which may be 

 called the anterior one. It finally settles down by the anterior 

 end and develops tentacles and a mouth at the opposite pole. 



