248 HYDROID POLYPES LESS. 



The medusae, when mature, become detached and 'swim 

 away from the hydroid colony. The sperms of the males 

 are shed into the water and carried to the ovaries of the 

 females, where they fertilize the ova, converting them, as 

 usual, into oosperms. 



The changes by which the oosperm or unicellular embryo 

 of a hydroid polype is converted into the adult are very 

 remarkable. 



The process is begun by the oosperm, still enclosed 

 within the body of the parent (Fig. 57, A), undergoing 

 binary fission, so that a two-celled embryo is formed (B). 

 Each of the two cells again divides (c), and the process is 

 repeated, the embryo consisting successively of 2, 4, 8, 16, 

 32, &c., cells, until a solid globular mass of small cells is 

 produced (D, E) by the repeated division of the one large 

 cell which forms the starting-point of the series. The embryo 

 in this stage has been compared to a mulberry, and is called 

 the morula or polyplast. 



So far all the cells of the polyplast are alike globular 

 nucleated masses of protoplasm squeezed into a polyhedral 

 form by mutual pressure. But before long the cells lying 

 next the surface alter their form, becoming cylindrical, with 

 their long axes disposed radially (F) . In this way a superficial 

 layer of cells, or ectoderm, is differentiated from an internal 

 mass, or endoderm. 



The embryo now assumes an elongated form (G) and 

 begins to exhibit slow, worm-like movements, finally escaping 

 from the parent and beginning a free existence (H). The 

 ectoderm cells are now found to be ciliated, and before long 

 a cavity appears in the previously solid mass of endoderm 

 cells : this is the first appearance of the enteron or digestive 

 cavity. In this stage the embryo is called a planula : it 



