O.V THE FOE'I'AL MEMBRANES OE I^HELOXIA 7 



Fii;-. 'A'2 is irniii the point where the lien<l-eiid «»1' the embryo is 

 }uM ])Vii\ii\\']])<x ^" ><\nk Inflow tlie level of the Idàstodcriii, Tlie meso- 

 hlast of the liody hns .^epiij-jited from the extr:i-emhryoTiie \mri . The 

 amiiioii is mostly epibl.istic, ;xlthoiio-h lined hy the hypolilasl iiir n 

 short distnnee on en<-li side. 



Imù". .').') is froiii the liead reo-ion whidi is completely snnk below 

 the level of tlie hinstoderm. As em|)hnsized bv Motfmnnn, the bead 

 aj)pears in the cr«»ss section Ix-low. instead of above, the blastoderm. 

 The aiiinioii is eom]>ose(l of the epiblast and hypoblast, eaeli heino- 

 only one-eell layered. 



These sections show that the amnion at this sta^'e eonsists, in 

 the region ol* the sunken head, of the epil)last and hypol)last, and in 

 the (h)rsal ren-ion, of the epil)last only. The mesoblast as vet has no 

 share in it. 



In the stajii'e with t! or 7 mesoblastie somites (Figs. )> and .') a PI. 

 I. See also Diag. TIF. and TIT. PI. X.), the amniotic hood has ex- 

 tended to the posterior end of the eml)i'yo, leaving (jnly the region 

 round the neurenteric canal exj)osed. The mesoblast has also very 

 much increased in its distribution and has become, thoughout, split 

 into the somatic and splanchnic layers. The cœlom has thus 

 appeared not only within the body of ihe cnibi-v«» proj)er but lias 

 extended itself into the cxti'a-embrNonic porti«)ji ol' the blastoderm. 

 Although the mesoblast has originally s])read IVom behind foi'ward, 

 the cœh^inic cavity appeal's first in the neck region of the embryo and 

 s])reads graduall}^ backward — as was pointed out by Strahl (No. 5). 

 In the stage represented in Pig. o, when seen through from above, 

 the extra-embryonic coelomic cavities of tAvo sides, extending into 

 the amniotic folds come close together (but are not fused) in the 

 median dorsal line along a considerable distance in the anterior part 

 of the dorsal region, but separate from each other before the posterior 



