EARLY DEVELOPMENT OF THE LACERTILTA. 647 



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

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

 gether. The epiblast in the primitive streak completely coales- 

 ces 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 (protovertebrae) 

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

 opaque embryo. 



The woodcut (fig. i) represents a diagrammatic longitudinal 

 section through this embryo, and the sections belonging to 



ne 



FIG. i. Diagrammatic longitudinal section of an embryo of Lacerta. //. Body 

 cavity, am. Amnion. ne. Neurenteric canal, ch. Notochord. Ay. Hypoblast. 

 ep. Epiblast. pr. Primitive streak. 



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

 and of the primitive streak. 



As is shown in fig. i, the notochord (c/i.) has now throughout 

 the region of the embryo become separated from the subjacent 

 hypoblast, and the lateral plates of mesoblast are distinctly 

 divided into somatic and splanchnic layers. The medullary 

 groove is continued as a deepish groove up to the opening of the 

 neurenteric passage, which thus forms a perforation in the floor 

 of the hinder end of the medullary groove (vide Series B, figs. 2, 

 3, and 4). 



The passage itself is somewhat shorter than in the previous 

 stage, and the whole of it is shown in a single section (fig. 4). 

 This section must either have been taken somewhat obliquely, 



