EARLY DEVELOPMENT OF THE LACERTILIA. 423 



mistaken, have overlooked its lower opening. As I hope to show 

 in the sequel, it is also the equivalent of the neurenteric passage, 

 which connects the neural and alimentary canals in the Icthjop- 

 sida, and therefore represents the blastopore of Amphioxus 

 Amphibians, &c. 



Series a, figs. 2, o, 4, 6, illustrate the features of the passage 

 and its relation to the embryo. 



Fig. 3 passes through the ventral opening of the passage. 

 The notochordal plate {c/i'.) is vaulted over the opening, and 

 on the left side is continuous with the mesoblast as well as the 

 hypoblast. Figs 3 and 4 are taken through the middle part of 

 the passage (ne.), which is bounded above by a continuation of 

 the notochordal plate, and below by the tissue of the primitive 

 streak. The hypoblast {/ly.), in the middle line, is imperfectly 

 fused with the mesoblast of the primitive streak, which is now 

 continuous across the middle line. The medullary groove has 

 disappeared, but the medullary plate (w^jy.) is quite distinct. 



In fig. 5 is seen the dorsal opening of the passage (?ie.). If 

 a section behind this had been figured, as is done for the' next 

 series (b), it would have passed through the primitive streak, 

 and, as in the chick, all the layers would have been fused 

 together. The epiblast in the primitive streak completely 

 coalesces with the mesoblast ; but the hypoblast, though attached 

 to the other layers in the middle line, can always be traced as a 

 distinct stratum. 



Fig. B is a surface view of my next oldest embryo. The 

 medullary groove has become much deeper, especially in front. 

 Behind it widens out to form a space equivalent to the sinus 

 rhomboidalis of the embryo bird. The amnion forms a small fold 

 covering over the cephalic extremity of the embryo, which is 

 deeply embedded in the yolk. Some somites (protovertebrse) 

 were probably present, but this could not be made out in the 

 opaque embryo. 



The woodcut (fig. 1) represents a diagrammatic longitudinal 

 section through this embryo, and the sections belonging to Series 

 B illustrate the features of the hind end of the embryo and of the 

 primitive streak. 



a^ffi 



Fig. 1.— Diagrammatic lougitudiual section of an embryo of Lacetra. 

 pp. Body cavity. am. Amnion. ue. Neurenteric canal. c/i. 

 Notochord. hi/. Hypoblast. ep. Epiblast. pr. Primitive 



streak. 



