OUTLINES OF THE DEVELOPMENT OF THE TU.A.TARA. 39 



In the head region, just outside the aortic dilatation on 

 either side of the front end of the alimentary canal, lies the 

 commencement of the head cavity. This is shown in fig. 62 

 as a small space in the mesoblast {H. C). Anteriorly it 

 extends forwards for a short distance, dilating somewhat and 

 becoming irregular, while posteriorly it soon disappears. 



The pericardium is now very largely developed, forming a 

 great quadrangular sac on the ventral aspect of the embryo 

 (figs. 59, 64 — 66, P. C). The cavity of the pericardium is 

 continuous above with the narrow coelomic space appearing 

 between the somatopleure and splanchnopleure in the body of 

 the embryo on either side of the alimentary canal (fig. 66). 



In the region of the heart (figs. 65, 66) the splanchnopleuric 

 and somatopleuric mesoblast on either side both turn inwards, 

 and unite ventrally with the corresponding layers of the 

 opposite side, the splanchnopleuric mesoblast thus enclosing 

 the heart, and the somatopleuric helping to enclose the peri- 

 cardium. Behind the pericardium the somatopleuric mesoblast 

 turns upwards with the epiblast to form the true amnion, 

 while the splanchnopleuric mesoblast runs out with the hypo- 

 blast to join the yolk-sac (figs. 55, 68 — 71). 



The Wolffian Bodies and Ducts. — These organs are 

 only just beginning to appeaf, and are as yet very ill-defined. 

 From about the fifth to the eleventh mesoblastic somites the 

 intermediate cell mass is thickened on its outer aspect (fig. 68, 

 /. C. 31.). Behind the eleventh somite this thickening seems 

 to be growing freely backwards between the peritoneal epi- 

 thelium and the epiblast as a solid rod of cells, the rudiment 

 of the Wolffian duct (fig. 69, TV. D.), which can be traced 

 backwards about as far as the thirteenth somite. Anteriorly, 

 opposite somites 6 and 7, segmental vesicles appear to be de- 

 veloping in the thickened intermediate cell mass (fig. 68, S. V.). 

 It would be beyond the scope of the present paper to trace the 

 development of these organs in detail, but it probably takes 

 place very much in the same way as described by Weldon for 

 Lacerta (6). 



Vascular System. — The heart (fig. 59, Ht.) has now the 



