238 



later found slightly in front of the embryo and often both on one 

 side of an extension of the median plane. This has been found by the 

 writer to hold good not only for Necturus but also for Amblystoma, 

 Rana, Bufo and Acris. 



There are undoubtedly objections to this method, since it is prob- 

 able that a puncture interferes, in a degree proportional to its se- 

 verity, with normal development. These disturbances are in the case 

 of delicate punctures so slight that their efiects are imperceptible. 

 The writer as well as others has often noted the position of the ex- 

 ovates with reference to certain peculiarities in the adjoining cells, 

 such as form, pigmentation, etc. In nearly all cases these marks 

 maintain a constant position with reference to the exovates. It would 

 seem therefore that the successive positions occupied by minute ex- 

 ovates during growth are indicative of the successive positions occupied 

 by a given embryonic area. 



Of course if the punctures are severe and the exovates large de- 

 velopment is much disturbed or even prevented. In such cases I see 

 every reason for the position taken by some writers, most recently by 

 Ikeda ('02), who concludes that the "normal course of development 

 cannot be made out from the results obtained by experiment". This 

 statement is readily comprehended from Ikeda's description of the 

 method which he employed, viz : "The injuries which I inflicted on the 

 eggs proved on the whole rather severe, but this was an advantage 

 rather than otherwise, for slight punctures heal rapidly and often leave 

 no trace, as has been observed by many investigators. Moreover as 

 already remarked by Assheton, punctures however slight have the 

 effect of causing eggs to deviate from the normal course, and if such 

 is the case it is preferable to have the deviations stand out un- 

 mistakably by making the injuries somewhat severe" (sic!). 



Some Remarks on the Formation of the Embryo. 



It has been shown that in various amphibia the basis of the head 

 end of the embryo is in the vicinity of the upper pole. In those eggs 

 which are more markedly holoblastic (Acris, Bufo, Rana, Amblystoma) 

 the head lies very near, if not at, the upper pole. In those eggs which 

 show a meroblastic tendency (Necturus) its basis is somewhat excentric. 



In all cases among the amphibia the head developes from some 

 part of the area at or near the upper pole of the egg in which local- 

 ity cell division has become most active. In other words the polarity 

 of the egg fixes the locality in which cellular activity is most marked, 

 and this area determines the locality of the head of the embryo. 



