DETELOPMENT OF THE CTENOPHOEA. 93 



when these have been nearly enveloped by tlie layer of small 

 perijiheral spheres, as usually presenting towards its centre a 

 more concentrated portion, in the form o£ a nucleus, from which 

 filaments radiate towards the circumference of the sphere ; but 

 he does not regard this as representing a true cell-nucleus. He 

 has noticed, however, a true cell-nucleus in each of the spheres 

 composing the enveloping layer. 



He has observed the formation of the swimming-plates, which 

 he describes as originating in series of cells, which at first project 

 hemispherically from the surface of the embryo. Each cell carries 

 on its projecting end a set of fine hairs, which ultimately become 

 fused together, and form the vibrating swimming-plate. "Will, 

 as cited by Chun, appears to have already noticed the formation 

 of the swimming-plates out of cilia thus fused together. 



The newest point ia Kowalewsky's memoir is probably his 

 account of the migration of cells from the surrounding parts into 

 the gelatinous substance which is to constitute the great mass 

 of the body in the adult. This jelly shows itself at first as a very 

 thin stratum between the peripheral layer of cells and the central 

 mass ; and he believes it to be formed at the cost of the large cen- 

 tral cleavage-spheres. It is at first perfectly clear and structure- 

 less ; but as it increases in volume some of the cells of the peri- 

 pheral layer detach themselves, and wander into the jelly. This 

 migration of the cells becomes more and more active. The mi- 

 grating cells at first project from the peripheral layer into the 

 jelly, into which they send off" pseudopodia-like prolongations, and 

 finally completely detach themselves, and move into the deeper 

 parts of the jelly, where their ofi'shoots freely anastomose with one 

 another. Treated Avith carmine solution, the homogeneous part 

 of the jelly becomes coloured, while the cells which had migrated 

 into it remain unafifected by the pigment. 



The observations of Kowalewsky aff'ord further evidence in 

 favour of the nervous character of the ganglion-like body which 

 lies under the so-called ocellus, with its capsule ; for he finds 

 filaments passing from it to be distributed to each of the series of 

 swimming- plates, every plate receiving a small branch from the 

 filament common to the entire series. The origin of the otolites 

 each in a cell, from which they escape and reach the interior of 

 the capsule, has also been noticed by him. 



Kowalewsky has also followed Cesium veneris through some of 

 the early stages of its development ; and he finds that the seg- 



