>48 



H. D. Senior. 



In Figs. 14 and 15 the heart has been almost entirely omitted, 

 the position of the venous orifice of the atrium (as it lies near the 

 surface of the yolk) is indicated by a broken line; the other struc- 

 tures shown are indicated as they would appear in a mid-sagittal 

 section of the embryo. 



Fig. 14 is a diagram made from the reconstruction used for Fig. 

 13, the left side is represented. The relations of the head-fold ecto- 

 derm to the surface have been approximately determined by com- 

 parison with earlier stages, and are indicated in the diagram. The 

 line of separation of head from yolk is indicated externally, in the 



Pharynx 



Fig. 14. — Diagram made from the reconstruction shown in Fig. 13 as seen 

 from the left side, x 1^0 diams. 



The venous end of th^ atrium is indicated by a broken line ; other structures 

 are represented as cut in the mid-sagittal plane. Ectoderm belonging to the 

 original head fold is shaded. The j^olk is stippled. 



embryo, by a U-shaped groove ; the anterior part of this groove (cor- 

 responding to the cross-piece of the U) appears in section in Fig. 14. 

 The anterior margin of the pericardial plate is attached to the 

 head-fold ectoderm and is, at present, dorsal to the groove referred 

 to (see Fig. 14) ; the lateral margins of the pericardial plate are 

 attached to the surface ectoderm just ventral to the groove. The 

 pericardial ccelom is, as in previous stages, dorso-ventrally compressed ; 

 three points in its wall will require to be noticed in the following 

 stages. The points referred to are : the anterior end, the posterior end 

 and the aortic root (apex of conus arteriosus) ; these are indicated, 

 respectively, by the letters X, Y and Z, 



