ON TEE FOETAL ICEMBRANES OF CHELONIA. 15 



The last named author ÇS(^. 8) hns also studied the process in the 

 chick and found it to he alike. My own ol)servations jiuive in all 

 essential points with the account gi\en hy these authors. The 

 process briefly stated is as folhjws : As stated before, the extra- 

 embryonic cœlomic cavities of the two sides become early united across 

 iîi the he;id re^'ioii, there being no sero-amniotic (.'onnection here. 

 This united caxitv or its niesol)hist \\;dl. in spreading itself, in- 

 sinuates itself hetween the epibhist and hyo]toblast of the bhistoderm 

 and thus |)Ushes the hy|>oljlast forward and downward. A com- 

 parison of Figs. 41 and 41(/ (PI. \'.) aii<l Diags. III., I\'., \'. (I 1. X.) 

 will make this jxnnt cI(Mu-. In Fig. 41. the head is still entirely 

 covered by tlie proamnion; in Fig. 4h/,, the extra-embryoiiic 

 cœlomic cavity (cad') in enlarging itself, has ])ushed the hypoblast 

 forward and peeled it otf, so to s])eak, from the greater part of the 

 proamnion covering the head, so that now tlie proamnion is found only 

 on the ventral part of the head. Meanwhile, the emliryo turning on 

 its longitudinal axis comes to lie on its left side. These movements 

 brinR- about the state of things as shown in F'in's. 11 and 11(/(P1. II.) 

 In Fig. 11 the end)ryo lies evitirely on its left side, and a small 

 anterior part of the head is covered by the now much reduced pro- 

 an.nion. In th(^ ventral \ iew of the same (Fig. IP/) the proamnion 

 is very conspicuous, because it is transparent and without blood- 

 vessels. A section from the head of this embryo is shown in Fig. 

 85 (PI. X.). Lt shows how the proamnion extends now only for a 

 short extent. 



The final disappearance of the proamnion is bn^ught about by 

 the continued extension of the mesoblast. Although the encroach- 

 ment of the proamnion takes place to some extent from behind and 

 before, it takes place most actively from tlie two sides. Pig- 86 (Id. 

 X.) is a se<'tion sinvilar to Fvj;. 85 Iron! a somewhat older embryo. 



