ZOOLOGY AND BOTANY, MICKOSCOPY. ETC. 569 



approximately circular tube to a structure of wide rectangular form, 

 with at first a slit-like lumen and finally a minute central aperture. The 

 cause of the change in shape lies in the conditions of growth, assisted 

 by the active cell-proliferation in the dorsal wall of the oesophagus. In 

 the sixteen-day embryo vacuoles begin to form in the lining epithelium 

 beyond the oral end, and the process of vacuolization continues until, at 

 the end of the thirty-second day stage, only the extreme oral end of the 

 oesophagus remains closed. The level of initial closure and of final 

 perforation are approximately the same — namely, the laryngeal level of 

 the oesophagus. The temporary atresia of the oesophagus in the Caretta 

 embryo would appear to be a device for the protection of the lung 

 during its development against yolk material from the gut. The 

 relatively frequent embryonic and congenital occlusions of the duodenum 

 just caudal to the pancreatic primordia may, the investigator suggests, 

 perhaps likewise find their explanation in terms of a phylogenetic 

 protective device against yolk invasion. 



Rate of Regeneration.* — C. Zeleny continues his study of the 

 factors controlling the rate of regeneration. The present paper 

 describes a fresh series of carefully controlled experiments. Frog 

 tadpoles were found more suitable for purposes of experiment than 

 salamander larvte, which tend to vary in rate of regeneration from day 

 to day, and the factor to which this fluctuation was due was not 

 discovered. The first series of experiments is directed towards com- 

 paring the rate of regeneration from old tissue with that from new 

 tissue. The whole of the data are given in tabular form. There was 

 little difference in rate between the two ; the slight difference is in 

 favour of the cut through newly regenerated cells, but it may not be 

 significant. As regards completeness of regeneration there was again 

 essential similarity between the old tissue and the new tissue levels. It 

 would therefore appear that the rate of regeneration is controlled in 

 large part by factors not inherent in the character or conditions of the 

 cells near the cut surface. In the case of the earliest stages, those in 

 which there is cell migration but no ceil division, there is some evidence 

 that the rate of regeneration may be greater from new than from old 

 tissue. 



The second series of experiments dealt with the effect of successive 

 removals upon the rate and completeness of regeneration. In experi- 

 ments in which the age factor was eliminated the second regeneration 

 had the advantage over the first, and the third over the second, both in 

 respect of rate and of completeness. In experiments in which the age 

 factor was not eliminated the time intervals varied somewhat in each 

 set, but there was evidently a decrease in rate from the first to the third 

 and fourth regeneration. The decrease is probably due to increase in 

 age, not to successive removals. 



In regard to the level of cut it was found as a general rule that the 

 length regenerated in a given time is proportional to the length 

 removed, or, in other words, the length regenerated per unit of removed 

 length is a constant. This, however, applies only to material produced 



* Illinois Biolog. Memoirs, iii. (1916) pp. 1-169. 



Dec. 19th, 1917 2 q 



