30 ARTHUR DENDY. 



accurately, as the axial portion of this forward growth sepa- 

 rated from the lateral portion ; while the hypoblastic lining of 

 the alimentary canal assumes its characteristic columnar form 

 from in front backwards. 



The rudiment of the pericardium, which should lie just in 

 front of the letters " jP. Sj)." in fig. 35, appears in this section 

 to be quite obliterated, but it is seen again in the correspond- 

 ing position at the next stage (compare fig. 50, P. C). 



Stage G (figs. 47—50). 



I have only a single embryo of this stage, numbered 59, 

 which was removed from an egg found on Stephens Island 

 about the end of November, and opened on December 9th. 



When examined as an opaque object this embryo was seen 

 to differ from those of the preceding stage chiefly in the 

 appearance of the ventral surface (fig. 47), which showed the 

 as yet unenclosed portion of the alimentary canal as a shallow, 

 elongated trough, distinctly bounded all round by a thick 

 bolster-like rim. Close to the hinder end of the oblong area 

 thus enclosed the aperture of the neurenteric canal was con- 

 spicuously visible (fig. 47, N. En.). The backward tilting of 

 the head (fig. 50) is more pronounced than in the preceding 

 stages, and at the same time the head is turned to one side 

 (figs. 48, 49) . 



The structure and relations of the amnion and pro-amnion 

 remain much as before. The serous envelope has progressed 

 greatly in development (figs. 47 and 50, S. En.), having been 

 formed almost entirely by the splitting off of the superficial 

 epiblast, accompanied by a thin layer of mesoblast, from the 

 underlying portion of the blastoderm. The latter may now 

 be termed the yolk-sac (figs. 47 and 50, Y. S.). In the re- 

 moval of the embryo from the egg the foetal membranes were 

 considerably disturbed, and the yolk-sac to a large extent torn 

 off around the embryo ; the result of this was to bring out 

 very clearly its relations to the overlying serous envelope, as 

 shown in fig. 47. Dorsally, of course, where the pro-amnion 

 joins the superficial blastoderm, the yolk-sac is reflected for- 



