3i6 ORR. [Vol. I. 



gland is the first to appear, and the lung the last. The head- 

 cavities have become large, oval-shaped cavities on either side 

 of the head, medial to the posterior part of the eye-cup (//^ 

 Fig. 2, C, PI. XII.). 



Figs. 2, C, and 24, C-42, C, represent that stage in which 

 all the embryonic organs have appeared save the lung, and the 

 point of origin of the latter is already indicated. The develop- 

 ment of some of these parts is given below in detail. 



2. The Nolo chord, Head-cavities, and Hypophysis. 



It is generally agreed that the notochord arises by a differen- 

 tiation of the linear median area of the dorsal wall of the primi- 

 tive intestine. The extreme anterior point of this differentiation 

 of hypoblast into notochord still remains uncertain for most 

 animals, and we are in doubt as to whether it remains constant 

 in its relations to other parts in all animals, or whether it is some- 

 times nearer to, and again sometimes farther from, the anterior 

 extremity of the intestine. In the frog, the intestine has the 

 appearance of extending much beyond the anterior end of the 

 notochord, while in the cyclostomata the converse seems to be 

 true, and the notochord appears to extend beyond the intestine. 

 The lizard, in this respect, seems to represent a primitive state, 

 for here the notochord continues to the anterior extremity of the 

 intestine, or the oral fusion of hypoblast and epiblast. The origin 

 of the notochord has already been referred to (S. i). In the 

 lizard it is relatively very much smaller than in Elasmobranchs 

 and Amphibians, and never attains such large proportions ; also 

 in relation to the growth of the other organs it becomes vacuo- 

 lated much later in the lizard than in Amphibians, In advanced 

 embryos the vacuolated part extends slightly anterior to the first 

 vertebral rudiment; but from here forward to the region of the 

 hypophysis it remains a long time in its primitive condition, 

 sometimes irregularly crooked, and finally disappears, leaving as 

 a last trace a more densely nucleated line in the tissue of the base 

 of the rudimentary skull. 



When the notochord first appears, its anterior part lies be- 

 tween the intestine and the brain-rudiment, touching both. Fig. 

 10, A, PI. XIII., represents a section tangential to the ventral 

 external surface of the mid-brain. In the third section, behind 



