OUTFJNES OF THE DEVELOPMENT OF Tllli: TUATAEA. 65 



figs. 22 — 32 aud fig. 35). It also takes place later in the 

 middle line, beneath the notochord, than it does laterally. 



(6) The Formation of the Foetal Membranes. 



The formation of the foetal membranes in the Tuatara presents 

 several very peculiar features, and appears to me to be by no 

 means of a primitive nature ; at the same time it presents no 

 features which are not met with in a more or less highly 

 developed state in other Vertebrate types. 



The Pro-amnion. — At Stage C, it will be remembered^ 

 the blastoderm of the area pellucida, forming the roof of the 

 large segmentation cavity (or subgerminal cavity) some little 

 way in front of the embryo, is thin and almost free from yolk 

 particles (fig. 4, Pro. Am.). This part of the blastoderm is evi- 

 dently to be regarded as the rudiment of the pro-amnion, but 

 we can hardly say that it consists, as in the chick, of epiblast 

 and hypoblast alone, with no mesoblast, for the lower-layer 

 cells still form a multiple layer, the lowest of which are only 

 just beginning to be doubtfully distinguishable as hypoblastic, 

 while the cells between these and the epiblast might certainly 

 be regarded as mesoblastic, though perhaps it is best to con- 

 sider the lower layer as being still undifferentiated into meso- 

 blast and hypoblast at all. We may therefore say that the 

 pro-amnion consists at this stage of epiblast and lower layer, 

 the latter being several cells thick. 



At Stage D this condition is still maintained, but behind 

 the pro-amnion, though still in front of the embryo, the hypo- 

 blast has become clearly differentiated, and a large ccelomic 

 space has appeared in the mesoblast (fig. 8) ; this, however, 

 does not concern us now. 



Between Stages D and E there is somewhat of a gap in the 

 series as regards the formation of the pro-amnion, but it is 

 evident that the head end of the embryo sinks down into the 

 yolk, carrying the pro-amnion with it. It thus comes to pro- 

 ject freely beneath the blastoderm, enveloped in a very thin 

 transparent membrane, the pro-amnion (fig. 22). This mem- 



VOL. 42, PART 1. NEW SEE. E 



