554 John Beard 



Segmentation is completed some few hours after fertilisation, but 

 the embiyo is stili enelosed in the vitelline membrane. Soon the epi- 

 blastic eells acquire cilia and the embryo rotates within the membrane 

 (fig. 12 . About twenty-four hours after the egg is laid the embryo is 

 hatched and the free-swimming larvai stages are entered upon. 



Larvai History. 



The larvai history may be divided into three well marked periods. 

 Firstly the period during which the larvai ciliation is not split up into 

 rings. This period extends over the second day after the egg is laid. — 

 Secondly, the period in which rings of cilia are dififerentiated, and in 

 which the larva is stili free-swimming. This period extends from the 

 end of the second to about the eighth day or thereabouts. Lastly, the 

 period in which the larva is no longer free-swimming but has its seat on 

 a Comatula. 



Earlier larvai Stages. — First period, 



When the embryo leaves the egg to enter upon its free larvai exis- 

 tence it is a minute, somewhat ovai, ciliated semitransparent being of 

 about 0, 05 mm in length (fig. 14). Its body is made up of an epiblast 

 of one layer of ciliated cells, and of certain larger and darker cells, 

 few in number, enelosed by the epiblast cells. These larger cells are the 

 future hypoblast and mesoblast (fig. 15). Alimentary canal and mouth 

 are not yet developed, and the blastopore has closed. A cuticle is 

 present, of a structureless nature, through which the cilia pass. 



The ciliation even at this early stage is not simple and uniform. 

 The larva figured by Metschnikoff is in this respect not correct. The 

 ciliation is difficult to make out, but appears to bave much the same 

 sort of peculiari ty of an-angement as in the adult. The cilia are not 

 uniformly scattered over ali the cells of the epiblast, but are arranged 

 in bunches or Clusters, and are somewhat stiff (fig. 14). Nor is it ar- 

 ranged over the whole body, forthe future anal region is destitute of cilia 

 (figs. 14 and 15). The larva soon lengthens, and assumes somewhat of 

 a pear shape (fig. 16). Towards the end of the second day, or early in 

 the third, the mouth is developed as an epiblastic invagination on the 

 site of the blastopore. This invagination grows inwards to meet the 

 stomach which has meanwhile been formed as the result of division of 

 the hypoblast cells. The mesoblast cells bave also probably increased in 

 numbers , and a little later furnish muscle-cells to the stomodaeum and 

 stomach, some portlons of the primitive mesoblast however stili remain 



