v ALTERXATIOXS OF GEXERATIOXS 321 



The oosperm undergoes segmentation, forming a poly- 

 plast or morula (p. 200) : ectoderm and endoderm 

 become differentiated, and the ectoderm-cells acquire 

 cilia, by means of which the embryo now swims freely 

 in the water. An enteron appears in the endoderm, 

 and in this stage the embryo, which has an elongated 

 form, is known as a planula. It then loses its cilia and 

 settles down on a rock, shell, sea-weed, or other sub- 

 marine object, assuming a vertical position, with its 

 broader end fixed to the support. 



The attached or proximal end widens into a disc of 

 attachment, a dilatation is formed a short distance from 

 the free or distal end, and a thin cuticle is secreted from 

 the whole surface of the ectoderm. From the dilated 

 portion short buds arise in a circle : these are the 

 rudiments of the tentacles : the narrow portion distal to 

 their origin becomes the hypostome. Soon the cuticle 

 covering the distal end is ruptured so as to set free the 

 growing tentacles : an aperture, the mouth, is formed at 

 the end of the hypostome, and the young hydroid has 

 very much the appearance of a Hydra with a broad disc 

 of attachment, and with a cuticle covering the greater 

 part of the body. Extensive budding next takes place, 

 the result being the formation of the ordinary hydroid 

 colony. 



Thus from the oosperm or impregnated egg-cell of the 

 medusa the hydroid colony arises, while the medusa is 

 produced by budding from the hydroid colony. \Ye 

 have what is called an alternation of generations, the 

 asexual generation or agamobium (hydroid colony) giving 

 rise by budding to the sexual generation or gamobium 

 (medusa), which in its turn produces the agamobium by 

 a sexual process, i.e., by the conjugation of ovum and 

 sperm (compare p. 287). This form of alternation of 

 generations is known as metagenesis. 



PRACT. ZOOL. 



