ON THE FOETAL MEMBliAXES OF CHELOXIA, 17 



Schliiiiclie " (Of. Struhl Xo. 5). The section« of this embryo show 

 that in the reo-jon where the hitenil fjlds of the amnion have not yet 

 united in (iie median line (Fig". 50 l*L \ I.), the fold is purely 

 e[)iblastie and sohd. and tlie mesol)hist has no share in it at all. 

 In the region where (he extra-emhryonie l)udy ea\"ity is jjresent (biii". 

 •M). tlie mesobhistie fokls have ah'eady pushed themselves eon- 

 sideraljly into the epiblastie amniotic sheet, dividing it int<» two 

 limbs: tlie amnion proper and the serous envelope. In the embryo of 

 Clem my s given in Fig. 2 (PI. I. ), the mesol)iast has as yet no share 

 whatever in the formation of the amnion. It follows therefore that 

 the mesublastic folds begin to push themselves into the epiblastie 

 amniotic sheet somewhat earlier in Trionyx than in Clenimys. 

 Fig. b'2 is lr(jm the head region of the embryo given in I'ig. 17. The 

 head is surrounded b^• the proamnion conn)osed for the most part of 

 the epi blast and hypoblast, and appears beneath the blastoderm 

 instead of aboN e it. 



Figs. 18 and U) (PI. III.) show that the amnion is gradually 

 spreading backward, and with it the extra-end)ryonic cœlomic cavities 

 are urowinn- laru'er and lart^er. In Fiu'. 18, the cavities of the two 

 sides are still wide apart over the doi'sal region of the embryo; in Fig. 

 11) they aluKjst touch each (jther along the median dorsal line in the 

 anterior dorsal pari of the endjryo. but are considerably apart ui the 

 posterior region. A section tivjni the anterior region (Fig. 57) shows 

 that thev are separated l)y the sero-amniotic connection which aj)j)e;u's, 

 however, still very short in a section. 



In the embryo given in Fig. 23 (PI. 1 \ .) the amnion has 

 covered the eml)ryo entirely and has even extended a short distance 

 behind it. The c(elomic cavités are correspondingly enlarged. 



In the stage gi\ en in Fig. 24 (PI. \\ .). the ])osterior amniotic 

 tube has become alread}' (piite elongated. Its ])os(erior opening is 



