OUTLINES OF THE DEVELOPMENT OF THE TCJATAKA. 69 



inner wall against the amnion above and the yolk-sac below, 

 the two being separated by a wide cavity containing the semi- 

 gelatinous liquid above mentioned. Before the close of de- 

 velopment the allantois spreads almost completely around both 

 embryo and yolk-sac, being interrupted at Stage R only by the 

 sero-amniotic connection above the embryo, and by a small 

 area at the opposite side of the egg where the serous envelope 

 has not yet split oiF from the yolk-sac. 



Marshall (7) describes the amniotic cavity of the chick as 

 forming a water-bath in which the embryo can move freely in 

 any direction. In the Tuatara this description is far more 

 applicable to the allantoic cavity, which vastly exceeds that of 

 the amnion in extent. 



(c) The Modelling of the Body and the Foundation 

 of the Principal Systems. 



The Modelling of the Body. — The body of the embryo 

 is first recognisable (Stage C) as a cap-shaped structure (figs. 

 1, 3, 5), the broad anterior end of which is raised up by the 

 head-fold, while the narrow posterior extremity, formed by the 

 primitive streak, lies at the general level of the area pellucida. 

 The general form of the embryo at this stage thus closely 

 resembles that of Trionyx, as figured by Mitsukuri and 

 Ishikawa (5). The anterior end of the embryo then elongates 

 and becomes at the same time narrowed (Stage D, fig. 6), and 

 as it sinks into the yolk, enveloped in the pro-amnion,it comes 

 to project freely beneath the overlying area pellucida (Stage 

 E, figs. 15 — 20). It remains in this position until Stage O, 

 when the front end of the embryo is withdrawn from the yolk- 

 sac, and comes to lie on its left side above it. 



A distinct tail-fold is not formed until much later than the 

 head-fold, but at Stage J (and perhaps even at Stage H) the 

 whole body of the embryo is clearly outlined by the head-, 

 tail-, and side-folds (figs. 56 — 58), and by the time Stage K 

 is reached the tail has begun to grow freely backwards between 

 the yolk-sac and the serous envelope (fig. 82). As the tail 

 elongates it becomes coiled inwards in a spiral between the 



