422 /a7//-:av7'.lvcj-: .i.vn devfj.opment 



theory of cytoplasmic localization will tiiul a substantial if somewhat 

 restricted basis. 



That we are here approaching the true explanation is indicated by 

 certain very remarkable and interesting experiments on the frog's egg, 

 which prove that each of the first two blastomeres may give rise either 

 to a half-embryo or to a whole embryo of half size, according to cir- 

 cumstances, and which indicate, furthermore, that these circumstances 

 lie in a measure in the arrangement of the cytoplasmic materials. 

 This most important result, which we owe especially to Morgan,^ was 

 reached in the following manner. Born had shown, in 1885, that if 

 frogs' eggs be fastened in an abnormal position, — e.g. upside down, or 

 on the side, — a rearrangement of the egg-material takes place, the 

 heavier deutoplasm sinking toward the lower side, while the nucleus 

 and protoplasm rise. A nezv axis is tJiiis established in tJie egg, which 

 has the same relation to the body-axes as in the ordinary develop- 

 ment (though the pigment retains its original arrangement). This 

 proves that in eggs of this character (telolecithal) the distribution of 

 deutoplasm, or conversely of protoplasm, is one of the primary forma- 

 tive conditions of the cytoplasm ; and the significant fact is that by \ 

 artificially changing this distribution the axis of the embryo is shifted. 

 Oscar Schultze ('94) discovered that if the Qgg be turned upside down 

 w^hen in the two-cell stage, a whole embryo (or half of a double 

 embryo) may arise from each blastomere instead of a half-embryo 

 as in the normal development, and that the axes of these embryos 

 show no constant relation to one another (Fig. 191). Morgan ('95, 3) 

 added the important discovery that either a half-embryo or a whole 

 half-sized dwarf might be formed, according to tJie position of tJie blas- 

 tomere. If, after destruction of one blastomere, the other be allowed 

 to remain in its normal position, a half-embryo always results,- pre- 

 cisely as described by Roux. If, on the other hand, the blastomere 

 be inverted, it may give rise either to a half-embryo ^ or to a whole 

 dwarf.* Morgan therefore concluded that the production of whole [ 

 embryos by the inverted blastomeres was, in part at least, due to a 

 rearrangement or rotation of the egg-materials under the influence of 

 gravity, the blastomere thus returning, as it were, to a state of equilib- 

 rium like that of an entire ovum. 



This beautiful experiment gives most conclusive evidence that each 

 of the two blastomeres contains all the materials, nuclear and cyto- 

 plasmic, necessary for the formation of a whole body; and that these 

 materials may be used to build a whole body or half-body, according 

 to the grouping that they assume. After the first cleavage takes 



1 Anat. Anz., X. 19, 1895. ^ Three cases. 



2 Eleven cases observed. * Nine cases observed. 



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