THE ORIGIN OF THE ORGAN-FORMING 

 MATERIALS IN THE FROG'S EMBRYO. 



T. II. MORGAN. 



The location of the neural plate of the frog's egg, and of the 

 organs lying immediately below it --the "embryo" in a narrow 

 sense - - has been variously determined by different embryolo- 

 gists. Pfliiger, Roux, Schultze, O. Hertwig, Morgan, Brachet, 

 H. V. Wilson, Assheton and others have considered this question, 

 but since the literature has been sufficiently reviewed in recent 

 years I shall refrain here from further comment. In the present 

 communication I shall deal with a problem somewhat different 

 from the location of the embryo, namely, the origin of the ma- 

 terials out of which the "embryo" is formed. If I can make 

 good my point it will be seen that the embryo-forming material 

 originates from a part of the egg different from that in which the 

 embryo first appears, hence the location of the embyro on the 

 egg does not give a sufficient answer to the problem of the real 

 source of the materials out of which the embryo develops. - 



In a series of papers, dealing with the relation between normal 

 and abnormal development of the frog's egg, I was led to certain 

 conclusions in regard to the origin of the organ-forming sub- 

 stances, and in the last paper of the series I dealt directly with 

 this question, basing my conclusions upon a reexamination of 

 normal development of Rana palustris. The present paper deals 

 with the same problem in Rana sylvatica and Bitfo lentiginosus, 

 and includes some new observations on Rana 



ARGUMENT. 



Briefly stated, my view is as follows : The material, out of 

 which the " embryo" develops, lies in the upper hemisphere, and 

 is transported below the equator of the egg during the segmen- 

 tation stages. This material forms a ring around the lateral wall 

 of the segmentation cavity, at first above, later below the equator 

 of the egg. The same material can be traced with some prob- 



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