59 



of two individuals head to tail, as it were, common in many bilocular Gre- 

 garinse (loc. cit,, figs. 18, 19), and in Monocystis also (fig. G), and Zygocystis 

 cometa of Stein, indicates a tendency to conjunction wbicU would favour the 

 enclosure of two individuals in a single cyst. At the same time it is only 

 right to state that tliis head-and-tail-attachment is commoner in small than 

 in large specimens of Gregarina, and may be due possibly to some process of 

 division occurring without encystment, though this is quite hypothetical. 



E. Ray Lankesteii, Ed. ' Q.J.M.S.' 



The Kinship of Ascidians and Vertebrates. 



In 1867 Kowalevsky published, in the transactions of the 

 Imperial Academy of St. Petersburg, some observations 

 which are more profoundly interesting than anything of the 

 kind which has appeared of late years, since, if correct, they 

 indicate distinctly a bridge over the chasm, suj)posed to 

 separate Vertebrates from all other animals. They prove the 

 existence of a bicavitary structure, of a neural tube and a 

 visceral tube, separated by an axial cartilaginous rod, in the 

 early stages of development of Ascidians, an arrangement pre- 

 viously believed to be essentially characteristic of Vertebrates. 



We are indebted to Professor Michael Foster for the 

 following account of Kowalevsky 's paper : — 



The observations of Kowalevsky were made on several 

 Ascidians ; but in all the processes are remarkably similar. 

 The earlier stages were chiefly studied in Phallusia mam- 

 miUata, the metamorphosis of the larva into the sessile form 

 in Ascidia intestinalis. 



The ova of Ascidians possess, on leaving the parent, a 

 somewhat complex structure. Each consists of a vitellus, 

 devoid of any proper vitelline membrane, and surrounded by 

 a layer of gelatinous material, in which are strewed yellow 

 nuclei or cells. These nuclei or cells are small homogeneous 

 vesicles, very similar in appearance to the blood-corpuscles of 

 the higher vertebrata, and are probably purely material 

 elements derived from the follicles in which the ova are 

 developed. Their subsequent history is remarkable, inas- 

 much as they become transformed into the '' white cells '^ of 

 the mantle. Outside this gelatinous layer is a hard mem- 

 branous capsule, which in turn is studded with peculiar 

 structures, varying exceedingly in the various species. These, 

 however, are of no importance and speedily disappear. 



The vitellus varies a good deal. In Phall. mammillata 

 it is highly transj)arent and refractive ; in Asc. intestinalis, 

 Cynthia, &c., it is opaque, brown or dark yellow. No nucleus 



