TBE DEVELOPMENT OF VERTEBRATA. 231 



enough to form the whole outer surface over the big mass 

 of yolk-cells, so supplementary epiblast cells are added to 

 them by cell-divisions in the outermost yolk-cells. This 

 process begins in the yolk-cells nearest the equator, which 

 adjoin the edge of the primitive epiblast, and gradually 

 extends to yolk-cells nearer and nearer the vegetative pole 

 (fig. 118, B,C). As the supplementary epiblast- cells at once 

 secrete pigment like that of the primitive epiblast-cells, and 

 as they resemble these also in size, it is easy to imagine that 

 the primitive epiblast is really growing and pushing its way 

 down towards the vegetative pole over the surface of the 

 yolk-cells. This, in fact, was formerly described in many 

 text-books as actually happening; but the process is now 

 known to be what has been described an extension of the 

 epiblast, not by growth, 

 but by addition of new 

 cells formed from the 

 yolk-cells. 



This extension of the 

 epiblast does not occur, A BO 



however, along quite *J* I^-FOKMATION OF BLASTOPOKE FBOO. 



11 *A i Diagrammatic postenor view. Epiblast black, 



the Whole OI its Original yolk-cells white. The neural folds are not 



edge At One point shown, although they would be seen in stage 0. 



which marks the future posterior end of the embryo, the 

 edge of the epiblast remains where it was. The down- 

 growing edges on either side of this point approach one 

 another and meet (fig. 119), so that by the time epiblast has 

 come to cover over the yolk-cells right down to the vegeta- 

 tive pole, there still remains a little circular area at the 

 posterior end where yolk- cells show at the surface. This is 

 the blastopore : it corresponds, not to the original wide 

 aperture of the archenteron in Amphioxus (fig. 112, B) but 

 to the small, posterior blastopore of the late gastrula 

 stage (fig. 112, c). 



4. The Epiblast of the Frog. Meanwhile the epiblast 

 has divided into two layers. This division occurs very early 

 in the primitive epiblast (fig. 118, A), and the new epiblast 

 formed from the yolk-cells soon follows suit. This is a 

 special peculiarity in the development of the frogs and toads 







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