10 K. MITSUKÜRI. 



across with each other over the dorsal region of the em- 

 bryo. A C(mnection — quite elongated and definite in 

 later stages — between the amnion and the serous en- 

 velope separates them to the very end of the develop- 

 ment. That this structure causes great peculiarities in the fœtal 

 membranes is to be expected and will become clear as later stages 

 are described. This connection, I shall call hereafter the sero- 

 amniot i c connect] on . It does not extend to the sunken head part 

 where the amnion cojisi.st.s of the ejnblast and hyp(jblast, and is con- 

 fined to the region l)ehind the neck representing the original solid 

 epiblastic sheet of the amnion oi- its prolongation behind. 



While Fig. 3 (PI. I.) Tio doubt represents the conunonest and 

 noi-nial form in which the amnion s])rea(]s backward, it seems by no 

 means to l)e the exclusive «jne. Fig. 14 (IM. II.) shows one in which 

 the posterior f)ld is present but a part of the left lateral fold is 

 absent, so that the hcjrse-shoe shaped postei'i<jr margin of the amnion 

 is open toward the left. I have also another embrjo in which a 

 part of the right lateral fold is absent. 



Now comes the most i'emarkal)le point in the development of the 

 amnion in C I em my s . According to what is hitherto known al)out 

 the amnion, one would expect that when it has reached the stage 

 shown in Fig. 3 (PI. 1.) the |)osterior fold will be produced or the 

 lateral folds will convei-ge toward each other and thus the amniotic sac 

 wnll be completely closed. Such is not the case in Clemmys. The 

 anteri(n- and lateral Iblds which stalling from the head have gradual!}^ 

 extended backward over the whole embryo do not stop at the 

 posterior end of the embryo but continue to grow back- 

 ward, although diminished in their width, until final- 

 ly there is produced a tube extending backward from 

 the posterior end of the embryo, almost as long as the 



