REPTILIA. 



2O9 



coincides with the line of the amniotic fold, and as this fold 

 rises up the two sides of the plate become bent over the embryo 

 and give rise to the inner limb of the amnion or amnion proper. 

 The section (fig. 127), representing the origin of the amniotic 

 hood of the head, shews very. well how the space between the 

 two limbs of the amnion is continuous with the body cavity. 

 The amnion very early completely encloses the embryo (fig. 128 

 A and B), and its external limb or serous membrane, after 

 separating from the true amnion, soon approaches and fuses 

 with the vitelline membrane. 



The first development of the allantois as a diverticulum of 

 the hypoblast covered by splanchnic mesoblast, at the apparent 

 posterior end of the primitive streak, has been described on 

 p. 207. The allantois continues for some time to point di- 

 rectly backwards; but 

 gradually assumes a 

 more ventral direction ; 

 and, as it increases in 

 size, extends into the 

 space between the se- 

 rous membrane and 

 amnion, eventually to 

 form a large, highly 

 vascular, flattened sack 

 immediately below the 

 serous membrane. 



The Yolk - Sack. 

 The blastoderm spreads 

 in the Lizard with very 

 great rapidity over the 

 yolk to form the yolk- 

 sack. The early ap- 

 pearance of the area 

 pellucida, or as it has 

 been called by Kupffer 

 and Benecke the embryonic shield, has already been noted. 

 Outside this a vascular area, which has the same function as 



md 



FIG. 130. ADVANCED EMBRYO OF LACERTA 

 MURALIS AS AN OPAQUE OBJECT 1 . 



The embryo was 7 mm. in length in the curled 

 up state. 



fb, fore-brain ; mb. mid-brain ; cb. cerebellum ; 

 au. auditory vesicle (closed) ; ol. olfactory pit ; 

 md. mandible ; hy. hyoid arch ; br. branchial 

 arches ; //. fore-limb ; ///. hind-limb. 



1 This figure was drawn for me by Professor Haddon. 



B. III. 



