No. 2.1 EMBRYOLOGY OF THE LIZARD. 321 



of the notochord and the coelenterlc zone in the embryos which 

 I have examined ; but as it is usual for degenerate rudiments of 

 organs to present variations, so also the anterior tip of the noto- 

 chord and the coelenteric zone apparently follow the general 

 rule. To follow the lines of development of these variations 

 would, of course, be impossible ; but the description of a few 

 stages will make clear the chief variations. In an embryo 

 sHghtly older than that of series B, and in which the mouth- 

 opening has broken through, I found the notochord anteriorly 

 widely separated from the hypoblast. Only the anterior tip 

 retains its fused connection with the roof of the hypophysis. 

 The notochord, for a short distance from the tip backwards, is 

 very crooked. Laterally from the tip of the notochord extends 

 the coelenteric zone, also connected for a distance with the 

 roof of the hypophysis. The lumen of the head-cavities extends 

 into the coelenteric zone very near to the median line, as is 

 shown by a sagittal section (47, PI. XV.), two or three sections 

 from the notochord. Other older embryos show that the coe- 

 lenteric zone has entirely disappeared, while the anterior tip 

 of the notochord still remains fused with the roof of the hy- 

 pophysis. This condition I found in one embryo older than that 

 of series C, and in another so far advanced that the head-cavities 

 had entirely disappeared. 



It might be supposed that in some cases the roof and pos- 

 terior wall of the hypophysis are formed from the hypoblast, 

 and that therefore the tip of the notochord has simply remained 

 connected with the cell-layer from which it originated ; but on 

 this hypothesis it would be difficult to imagine an adequate 

 reason why the tip should be bent out of line to retain this con- 

 nection. The appearance of the hypophysis in all the embryos 

 is so similar, that there is no reason to suppose a variation in its 

 origin. In one instance I have shown it to be derived from the 

 epiblast ; this agrees with what has been found by authors on 

 the embryology of other vertebrates. In the present case, the 

 first appearance of the tip of the notochord, the coelenteric zone, 

 and head-cavities is in the form of a small mass of cells, ap- 

 parently budded from the hypoblast. This mass is fused with 

 the epiblast. In some individuals the notochord and coelenteric 

 zone separate from the epiblast at the same time, though retain- 

 inc: connection with each other. In other individuals the coe- 



