HARDWICKE'S SCIENCE-GOSSIP. 



127 



conclusion that they were of a truly choleraic nature. 

 They, however, clearly show one thing, for whether 

 the disease were choleraic or not, there can be ab- 

 solutely no doubt that choleraic commas were present 

 within the intestinal canal in large numbers ; and, 

 therefore, that their presence there in any case 

 cannot be taken as conclusive evidence that this 

 was the site of primary invasion of the system by 

 the parasite." 



The experiments reviewed above were conducted 

 by Dr. D. D. Cunningham. The importance of my 

 extract must be my apology for its length. 



W. J. Simmons. 



Calcutta. 



THE DEVELOPMENT OF THE TADPOLE. 



HAVING had an opportunity last month of 

 watching day by day the development of this 

 animal, from the first segmentation of the egg to the 

 full-grown tadpole, I paid particular attention to the 

 first appearance and disappearance of the cilia in the 

 epidermis, the presence of which I mentioned in a 

 note on p. 187 of last year's volume, and am now able 

 to give some further particulars on the subject. 



I obtained some frog's ova on the 17th of March 

 which had just been laid, and saw the breaking up of 

 the large primitive cell into two, four, eight, sixteen, 

 etc., segments, until a vast number of small cells were 

 produced, and by the next day the morula stage of 

 the embryo was completed. On the third day, a 

 semicircular depression indicated the ingrowth of the 

 epiblast and formation of the blastopore, which was 

 completed the same evening. On the fourth day, the 

 first indication of a dorsal depression, the primitive 

 groove, giving rise to the brain and spinal cord, be- 

 came visible, and had made considerable progress by 

 the evening of the same day. On the fifth day, the 

 medullary folds closed over the primitive groove, and 

 the line of contact became almost obliterated. An- 

 teriorly, a median depression indicated the formation 

 of the future mouth, and below this were two lateral 

 depressions for the suckers. One blastopore was still 

 visible as a small pore, but no indication of the tail 

 was as yet visible. 



At this stage, and on the evening of the fifth day, 

 the cilia made their appearance. I saw the very 

 minute cilia distinctly, when their motion was still 

 very weak ; but shortly afterwards they could no 

 longer be seen, on account of their rapid motion. 

 Their presence was, however, clearly indicated by 

 the strong current in the fluid surrounding the embryo 

 within the egg membrane, which current did not exist 

 before. On the following day, the cilia had become 

 much larger, and by careful adjustment of the light, 

 I have seen them every day until the twenty-eighth 

 day, when they finally vanished. 



The young tadpoles emerged from the egg on 1 he 



seventh day, attaching themselves with the suckers 

 to the gelatinous egg-shells ; and before they could 

 swim the action of the cilia was sufficient to propel 

 them slowly through the water. 



The cilia do not cover equally the whole epidermis, 

 as would appear at first sight ; but a number of single 

 cells only acquire cilia, and these are separated from 

 each other by three or four cells which do not bear 

 cilia. As the embryo grows, the distance between the 

 ciliated cells increases somewhat, and the appearance 

 of " tufts " of cilia is produced. 



The cilia are first lost on the tail. On the twenty- 

 fifth day, they were still very active ; but after that 

 day they gradually disappeared, and on the twenty- 

 eighth day I saw the last trace of them, being twenty- 

 three days after their first appearance. 



I have also seen the cilia with high powers in 

 various ways — by crushing the embryo, making rough 

 sections with the scissors, etc. But the most satis- 

 factory way of seeing them is to proceed as follows : 

 Kill the young tadpole by cutting off his head ; then 

 cut off his tail, and place the latter on a glass slide in 

 a drop of water ; add a drop of stain, carmine, or 

 logwood ; put on a cover glass, and examine at once 

 with \ or \ in. object glass. All round the edge of the 

 tail the tufts of cilia will be seen still vibrating feebly. 

 In this way I have measured them, and find their 

 length to be about ^5 in. I have mounted a tail in 

 balsam, after hardening in chromic acid, and it shows 

 the cilia perfectly. 



I have also demonstrated the vibrating cilia to the 

 satisfaction of the president and members of the 

 Quekett Microscopical Club, at the March meeting. 



As the development of the tadpole is accelerated 

 by warmth and retarded by cold, I must add that I 

 kept them in a small aquarium in my room, where 

 the temperature varied between 14 and 18 centi- 

 grade. 



Charles Rousselet. 



THE EYES OF THE WATER-FLEA. 



A LL eyes, however complex, are developed from 

 I~ *• a portion of the epithelium of the skin. All 

 that is required for the formation of an eye is a mass 

 of pigment, with a transparent refractive body to 

 concentrate light on it, and a nerve to bring the pig- 

 ment into relation with the central nervous system of 

 the animal. 



When light falls on animal pigment, some change, 

 either chemical or physical, occurs in that pigment, 

 and the conscious effect that this produces in the part 

 of the nervous centre set apart for its reception, is 

 called vision. 



An eye may be formed in any part where there is 

 epidermal epithelium. In the leech, for instance, it 

 is found that in several parts of the body individual 

 cells of the skin are modified so as to form lenses, 



