It is evident, (hen, that in the jouiig stages, Just referred to, the 

 pglarily of the cell iia^? not yet been determined. It also appeared 

 from the experiments tliat the determination takes place during the 

 gastrulatioii. If the blastopore is still like a straight or slig-^htly 

 crescent-shaped slit, the future ectodermal ciliated cell is still indifferent. 

 But as soon as the blastopore has become horseshoe-shaped and still 

 )|kter circular, reversion of ectoderm without reversion of the future 

 direction of the ciliary- movement is not possible. 



It follows, then, that the period of determination of the polarity 

 of the epidermal ciliated cell falls in an e^rly stage of gastrulatioii. 



Now we bad to ask if the determination of tiie polarity of the 

 cell coincided with the or'gan-determinatioii. 



To ascertain this we interchanged patches of presumptive e|tiderrais 

 and presumptive medullary plate in very young larvae, and we 

 watched the subsequently developing ciliary movement while giving 

 due attention to the original [)osition (vital staining). Stated briefly 

 the results were to the following effect : When after the operation 

 larvae appeared with a normally developed medullary plate (part 

 of which was consequently generated by presumptive epidermis) and 

 with a normally developed epideruiis (part of which was consequently 

 formed by presumptive medullary plate), the larvae exhibited normal 

 direction of ciliary movement i.e. the ciliated cells have not developed 

 as they would ha^'e done originally, but have adapted themselves 

 to theii' new eu\ iroiimeiit. If the organ-determination has not yet 

 been effected, the direction of the ciliary movement can still be 

 influenced by the environment. But if abnormal larvae developed with 

 a deficient medullary plate (jr with pieces of the medullary plate 

 in their epidermis, then the diTeXTttöü of the movement appeared to 

 have developed on the implantates according to the origin of the 

 implantates and appeared not to have been influenced by the new 

 environment. 



Our experiments, therefore, seem to im|ily that the determination 

 of the polarity of the cells and of the organogenetic function either 

 occur synchronously or at all events with a very brief interval of 

 time. 



It should be borne in mind, however, that the organ-determina- 

 tion in the ectoderm does not occur everywhere at the same time. 

 Spkmann's ami Mrs. Mangold-Pröschhoi,d's ') experiments have shown 

 that this determination starts from what they have termed an , .or- 

 ganisation centre", which is located in the dorsal lip of the blastopore. 



H. Spemann. AilIi. f. Enlw. iiiech. der Organismen. Bnd. 48. 1921. 



