12 K. MTTSUKURI. -j 



posterior amniotic tube seems to keep pace. 



The condition of the amniotic sac proper at this stage when the 

 posterior tube has ah'early been developed is shown in the series given 

 in Figs. 42-47 (1^1. VI.) from an emln-yo with twenty mesoblastic 

 somites. An inspection of these figures shows rliat. over the ])osterior 

 part of the embryo (Figs. 42 and 43), the amnion and the serous 

 enveh)pe are still adhei-ent to each other for a considerable space : 

 hence the extra-embryonic rœlomic cavities (cœl') of the two sides are 

 separated from each other by a wide interval over the dorsal region. 

 As we proceed forward, the mesoblastic folds gradually push toward 

 the median line se])arating the amnion from the serous envelope, until, 

 over the middle region of the embryo, they are separated only by a 

 thin partition (Figs. 44 and 44a). This partition — the sero-amniotic 

 connection^ — -has now become vertically somewhat elongated and unlike 

 Figs. ?)(vi and b (PI. V.) ])resents a string of cells in a cross-section 

 (Fig. 44a). This represents the greatest vertical elongation of the sero- 

 amniotic connection at this statte. Further forward, the mesol)]astic 

 folds becoîne again separated by a considerable interval (Fig. 45). An- 

 teriorly to the point wluu-e the head-end l)egins to sink beneath the 

 surface of the l)lastederni, the cœlomic cavities of two sides which 

 arose separately have l)econie united across, there being no sero- 

 amniotic connec'tion from the beginning in this part (P'ig. 46). In the 

 head which is freely pi-ojecting into the cavity below the Ijlastoderm, 

 the amnion still consists only of the e])iblast and hypoblast (Fig. 47). 



Fi'oin what has been given above, it iollows that the extra- 

 embryonic cœlomic cavities of two sides are separated from each 

 other over the dorsal median line by the seio-amniotic connection 

 from t\\c n«M'k region to the very tip of the posterior 

 tube. In front of the neck region, i.e., in the siuiken head regicm, 

 the cavities become early united across. It is important to 



