OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 55 



meiit of this stage little or no ossification has taken place, this 

 process has progressed very considerably before its close. 



This stage is a very good one at which to describe the con- 

 dition of the foetal membranes at what is probably almost, if 

 not quite, the acme of their development. The following 

 account is based chiefly upon dissection of the membranes in 

 Nos. 143 and 149, verified by dissection of others. 



The embryo lies on its left side and lengthwise in the shell, 

 and by opening the eggs carefully under water, in which they 

 sink, it was found that there is no air-chamber. 



On piercing the shell carefully a small quantity of a thin 

 colourless liquid is squirted out with some force, showing that 

 there must be considerable tension within the egg. This liquid 

 corresponds to the '* white" of a hen's egg, but in the Tuatara 

 egg, as already observed, it forms only an extremely thin layer 

 between the shell and the serous envelope. 



The serous envelope extends all round inside the shell. 

 Above the embryo it is still connected with the true amnion 

 over a small area in the region of the shoulders, and opposite 

 to this, on the other side of the egg, it becomes continuous 

 with the yolk-sac over another small area. 



The true amnion closely envelops the entire embryo as a 

 thin transparent investment. 



The allantois is very largely developed, and extends almost 

 completely around the embryo inside the serous envelope. It 

 is filled with a viscid, semi-gelatinous, transparent, colourless 

 liquid, closely resembling the white of a hen's egg. By the 

 presence of this liquid it is greatly distended, so that its outer 

 limb is pushed closely against the serous envelope, while its 

 inner limb is pushed close against the true amnion above and 

 the yolk-sac below. I found it impracticable to pierce the egg- 

 shell without piercing the serous envelope and the outer limb 

 of the allantois at the same time, so that the emission of the 

 thin liquid " white '' is immediately followed by a copious 

 oozing out of the viscid contents of the allantois. The tension 

 being thus relieved, it is possible to remove the shell without 

 further injury to the foetal membranes. 



