PHYLUM CCELENTERATA 



141 



from the endoderm or (more usually) ectoderm cells of the gonad ; 

 but in many cases originate in the coenosarc, and slowly migrate 

 to their final destination in the gonad, where they metamorphose, 

 in the usual way, into the definitive reproductive products. 



The development of the Leptolinas frequently, but not always, 

 begins within the maternal tissues, i.e. while the oosperm or im- 

 pregnated egg-cell is still contained in the gonad of the medusa or 

 in the sporosac. The oosperm divides into two cells, then into 

 four, eight, sixteen, &c. Fluid accumulates in the interior of the 

 embryo, resulting in the formation of a blastula or hollow globe 

 formed of a single layer of cells (Fig. 97, A). The blastula 

 elongates, and the cells at one pole undergo division, the daughter- 

 cells passing into the cavity, which they gradually fill (B). At 



FIG. 98. Early development of Eucope. A, blastula^-stage ; B, planula with solid endoderm ; 

 C, planula with enteric cavity ; al. enteric cavity ; ep. ectoderm ; hy. endoderm. (From 

 Balfour's Embryology, after Kowalevsky.) 



this stage the embryo is called a planula : it consists of an outer 

 layer of cylindrical cells the ectoderm which acquire cilia, and an 

 inner mass of polyhedral cells the endoderm. In some cases the 

 planula arises by a different process : a solid morula is formed, the 

 superficial cells of which become radially elongated and form 

 ectoderm, the central mass of cells becoming endoderm. By 

 means of its cilia the planula swims freely, and before long a 

 cavity appears in the middle of the solid mass of endoderm, the 

 cells of which then arrange themselves in a single layer around 

 the cavity or enteron (C, al). The planula then comes to rest, fixes 

 itself at one end to some suitable support, and becomes con- 

 verted into a simple polype or hydrula by the attached end 

 broadening into a disc and the opposite extremity forming a 

 manubrium and tentacles. The hydrula soon begins to send off 

 lateral buds, and so produces the branched colony. 



