Envelopes 



165 



extra -embryonic blastoderm gives rise to the envelopes of 

 tlie embryo. 



The tail-end, which is particularly thick in this stage, 

 now bends inwards (i. e. towards the centre of the egg) 

 and a little backwards (i.e. towards the hinder pole of 

 the egg), pushing before it the sexual germs, which are, 

 so to speak, caught in its 



/■■■ •• \f J/ ■:'<=i.,V\ 



concavity (fig. 122). 



The extra - embryonic 

 dorsal blastoderm now 

 sends out a fold (tail-fold 

 of the amnion) which 

 grows backwards in close 

 contact with the embryo 

 as far as the hinder pole 

 of the egg, and ultimatel}^ 

 still further, bending round 

 to the ventral surface. A 

 little later a head-fold 

 forms just behind the head 

 from the same dorsal blas- 

 toderm, pushes forward, 

 and then, curving round 

 the anterior pole of the 

 egg, grows backward along 

 the ventral surface to meet bryo dm- 

 the tail-fold. The _„ „'. 



^ ^ ^ , , p termination of tail-fold upon future ven- 



lolds coalesce, and lorm a u-a\ sm-faoe. ua. hind-sut. hi, sexual 



Fig. 



-Lor^itudiual section of em- 

 ■ formation of tail-fold of am- 



nion. /(, head, vj)^ ventral plate, am, outer 

 two layer of amnion. am\ inner ditto, am" , 



tral surface. luu hind-gut. .'</, 

 , , -, 1 • ^ ^ , o-erms. (From Kitter, 1890, fig. 19.) 



double embrj^onic mem- 

 brane, which is singularly like the amnion of the higher 

 vertebrates (fig. 123). 



The coalescence is not at first quite complete, for an 

 oval space on the ventral wall remains for some time 

 uncovered by the amniotic folds. The edges of the folds 

 slowly extend until the whole embryo is enclosed. 



