DEYELOPMENT OF THE CTENOPHOBA. 101 



enlarging and assuming a cylindrical form. Upon some of these 

 cells may soon be seen a number of exceedingly fine cilia, whicb 

 almost at their first appearance unite with one another, and form 

 the embryo swimming-plate, as first observed by Will and after- 

 wards by Kowalewsky. Each swimming-plate is thus the produce 

 of a single cell. 



Chun was the first to call attention to the fact that the entire 

 ectoderm of the embryo is clothed with exceedingly minute 

 vibratile cilia. Of these cilia eight linear series become more 

 strongly developed simultaneously with the appearance of the 

 swimming-plates, extend from the aboral end of every swimming- 

 plate series towards the sense-pole, unite two by two, and finally 

 end in the base of four strong sabre-shaped oscillating cilia. 



When the ectodermal layer has closed over the gastrula-mouth, 

 the cells composing it here multiply by division over a small 

 polar area, and at the same time become elongated to about twice 

 their original height. The sharply circumscribed ectoderm-region 

 thus formed becomes somewhat rounded, sinks a little below the 

 surface, and forms the central nervous system. Tw^o semicircular 

 groups of ectoderm-cells may now be seen, one on each side of 

 the central nervous system. These soon acquire an elongated 

 oval form, with the long axis in the plane of the stomach, and 

 become the "pole plates." Surrounding the central nervous 

 system may now be seen four groups of strong cilia. These in- 

 crease in height, unite with one another so as to form a closed 

 cilia-wreath round the nerve-mass ; and finally the individual cilia 

 become fused together and arch over the nerve-mass in the form 

 of a closed capsule. 



At an early period strongly refringent corpuscles may be 

 noticed in many of the nerve-cells. These are finally expelled 

 from their generating cells and become agglomerated into an 

 otolite-heap, which lies in the interior of the nerve-capsule, where 

 it is driven from place to place by the action of the cilia, which 

 line the floor of the capsule, until it finally adheres to the curved 

 ends of the four large sabre-shaped cilia already mentioned. The 

 otolites are composed of phosphate of lime. 



The central nervous system thus, of all organs of the Cteno- 

 phora, attains earliest its definitive form. Through its intimate 

 relation to the swimming-plates it is among the most charac- 

 teristic organs of the Ctenophora, and is found in all the species 

 with great uniformity. 



