CHAPTER XXXIV. 



EXTRA-EMBRYONIC MEMBRANES 



It will be remembered that when the mesoderm splits into a somatic 

 and a splanchnic layer, it extends out over the yolk so that there is no 

 definite line of demarcation separating embryo from the surrounding 

 media. First, the head fold appears, delimiting the embryo at the 

 cephalic end, and later the tail-fold and lateral fold do the same for the 



pi bl - 



Fig. 286. 



A, B, C, D, four stages of development of the embryonic membranes in birds. 

 al, allantois; am, amnion, (in Fig. B., this forms folds which give rise to both 

 amnion and serosa) ; am.h., amniotic cavity; d, digestive cavity; do, yolk-sac. 



E. Cross section through entire egg (including shell), all, allantois which 

 begins developing at the blunt end of egg ; am.h., amniotic cavity ; coel.ex., extra- 

 embryonic coelom ; do.s., yolk-sac showing development of mid-gut do.s.d. ; do.h., 

 covering of yolk ; l.k., air-chamber ; mes.w., extensions of the mesoderm between 

 the communicating opening of yolk sac and amniotic cavity pl.bl. The remaining 

 portion of the yolk covering (do.h.) closes the passage. These mesodermal exten- 

 sions as well as the lower tips, at the pointed end of the egg, close later and thus 

 form a closed amniotic cavity, pl.bl., amniotic cavity which develops from the 

 ectoderm with tiny projections on the inner side. It is in this cavity that the 

 remaining yolk is found ; pl.bl.-do.s., Communicating passage between amniotic 

 cavity and yolk-sac. (A, B, C, D, after Boas, E, after Duval.) 



caudal and lateral regions, so that after these folds have bent downward 

 and under the embryo, and almost separated the embryo from the yolk, 

 we speak of the space between the somatic and splanchnic layers as the 

 intra-embryonic coelom and the extra-embryonic coelom, according to 

 which portion lies within, and which portion lies outside, the embryo. 



The limiting folds which are continuous with the head fold and ex- 

 tend on each side of the embryo as the lateral folds, form the line of de- 



