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of sex-cells in several sections on each side of the gap, multiplied by 

 the number of missing sections. A comparison of these two embryos 

 with embryos 10 and 11 of the same nest, shows that the number 

 of sex-cells in each is intermediate between the totals of the latter two. 



We have in these figures the clearest evidence that in the case 

 of double monsters of this kind, arising as they do from the same 

 egg, each of the embryos has a normal number of sex-cells. The sum 

 total of the pair in question is far above the number of sex-cells 

 counted in any other embryo, and is about twice that of an average 

 struck between the other embryos from the same nest. 



It will be remembered that Boveri discovered in Ascaris that 

 one of the cells of the two cell stage loses certain parts of its chro- 

 matin material and gives up all power of forming sex-cells — that it 

 becomes differentiated as a somatic cell. The other, however, gives 

 rise to both sex-cells and somatic cells. Reasoning from experiments 

 upon many invertebrates, notably the sea-urchin, many have been led 

 to believe that identical twins arise from the independent development 

 of each blastomere of the two cell stage. It is obviously impossible 

 to apply both views to the turtles unless we assume that these two 

 double embryos arose from binucleate eggs in which each nucleus 

 would act as an independent center of development. This hypothesis 

 would seem very improbable in the light of the fact-well known to 

 all who have studied the development of vertebrate ova — that while 

 young oocytes may be bi-nucleate or pluriuucleate, the extra nuclei 

 invariably degenerate, leaving but one. We can not hope, however, 

 to reach positive conclusions upon these points while there are so 

 many unknown quantities in the problem. 



Beard, '02, stated that he had two pairs of Elasmobranch em- 

 bryos (Raja batis?) in each of which there were two individuals upon 

 a single blastoderm. In that article, he hinted at important results 

 expected from a study of these pairs, but he has as yet, so far as I 

 am aware, given no account of them. Unless we accept the very 

 slender possibility that these embryos may have developed from a 

 binucleate egg, we are forced positively to repudiate Beard's hypo- 

 thesis regarding the origin of each embryo from one of the primary 



2n 2 



sex-cells. His formula would have to be represented by — - — , which 



manifestly does not apply to the case of my specimen — embryos 

 Nos. 8+9. In addition to this, we have the enormous amount of 

 variation in the total number of sex-cells. From the results here set 

 forth, it would appear that the number of sex-cells found during this 



