246 H. D. Senior. 



cally complete, caudal vein is received entirely by the cardinals. 

 The subintestinal vein has lost all connection with the caudal and 

 the cardinals, and is now the portal vein. 



Stage of 6.2 mm. 



Fig. 13 represents the ventral surface of a reconstruction of the 

 pericardial region of an embryo of 6.2 mm. ; the reconstruction 

 extends further forward than those shown in the previous figures ; it, 

 in fact, includes the whole of the anterior part of the head which 

 remains at this time in contact with the yolk. The ventral wall of 

 the pericardial coelom has been partially removed to show the con- 

 dition of the heart-tube. The conus, ventricle and atrium are now 

 quite distinct, but there is no prominence on the ventricle correspond- 

 ing to its future apex. There is evidence, at this stage, that the 

 ventral wall of the pericardial ccslom is attached, rather extensively, 

 to the yolk just to the right of the venous orifice of the atrium. 

 Over the area of attachment no endocardium is present ; the exact 

 distribution of endocardium over the remainder of the ventral wall 

 of the pericardial coelom is difficult to make out owing to the ex- 

 treme tenuity of the latter. 



The venous end of the atrium has moved forward and is now placed 

 ventral to the posterior half of the left eye (it was altogether pos- 

 terior to the eye in the preceding stage) ; the anterior end of the 

 pericardial plate has moved forward even more than the atrium 

 and now appears to have reached the anterior limit of the head-fold. 

 There are several changes going on, however, which tend to com- 

 plicate matters by shifting former landmarks ; these changes can 

 be partially appreciated by reference to Figs. 13 and 14. In the 

 first place the head is rising from the yolk: this is accomplished 

 by forward growth of the head, by shrinking of the yolk, and by a 

 horizontal separation of the head-fold into its original two layers. 

 The dorsal and ventral layers of the head-fold are now being added 

 to the surface ectoderm (basal layer) of the continuous regions of 

 head and yolk respectively. In the second place the, now separating, 

 head-fold is moving bodily backwards so that it approaches the man- 

 dibular pouch; the latter shows evidence of antero-posterior com- 



