703 



therefore, closely examined and represented in diagrams. It was 

 fnrlher established that the thiid-slreams flow invariably inlhesanie 

 direction. A reversed direction ot the ciliary movement seems to 

 have rarely been observed in metazoa. (Erhard)'). 



This implies such a structure of the ciliated cells that a ciliary 

 movement is only possible in one direction, the cells present a 

 certain asymmetry in their structure; besides their polarity (by 

 which base and ciliated free surface are distinguished) tiiere is an 

 "accessory polarity" (vide Roux') foi' these ideas). The question has 

 been considered whether this accessory polarity could be reversed 

 artificially, in other words, whether the ciliated cell could by some 

 artificial method be made to move in the opposite direction. This 

 question is connected with another, viz. in how far the ciliary 

 movement depends on the position of the ciliated cells relative to 

 the axis of the body. 



Experiments performed by v. BRtJCKE') and those made this very 

 year by Merton') bear on this question. They did not succeed in 

 bringing about a reversion of the polarity. Now it has been evidenced 

 by numerous experiments that in the embryonic development there 

 is a period in which the ectoderm, from which the larval epidermis 

 is derived, is still indifferent. Spemann ') e.g. found that at the beginning 

 of the gastrulation ectoderm, destined to build up the medullary plate (so- 

 called presumptive medullary plate), could be replaced by presumptive 

 epidermis. Larvae developed with normal medullary plate and normal 

 epidermis. The fate of the ectoderm-cells in that stage of develop- 

 ment has^ not been, or has not yet been determined. The ectoderm 

 is still in a high degree liable to change (,,umbildungsfahig"). 

 Whether in that phase it is still completely indifferent cannot be 

 decided without a detailed inquiry. It occurred to me that an inquiry 

 into the polarity of the cell might afford some indication, as the polarity 

 of the cell may already be determined before its organogenetic function. 

 Spemann's experiments regard the organ-determination. Now, how about 

 the polarity of the cell? When is it determined? The experiments 

 in which I tried to solve these questions, I peiformed on larvae of 

 Rana esculenta and of Triton alpestris in the Zoological Institute of 

 the Freiburg University (Director Geheimrat Prof. Dr. H. Spemann). 



') Erhakd in Abderhalden's Handbuch der biologischen Aibeitsmethoden. 

 ') W. Roux. Terminologie der Entwickluugsinechanik der Tiere und Pflanzen. 

 Leipzig. Engelmann. 1912. 



') E. Th. V Brücke. Pfliiger's Archiv. f. d. ges. Physiol. Bnd. 166. 1917. 



♦) H. Merton. Pnüger's Archiv. f. d. ges. Physiol. Bnd. 198. 1923. 



") H. Spemann. Sitzungsber. d. Gesellsch. naturf. Kreunde. Berlin. 1916. N*. 9. 



