230 H. D. Senior. 



blend, enclose a circular area which contains the cells about to form 

 the root of the aorta (see Fig. 7D). Where blending has occurred 

 (Figs. 7B and 7C) continuity is established between the somatic 

 layers of the right and left lateral plates; the splanchnic mesoderm' 

 of the two sides becomes continuous across the mid-line in a similar 

 manner. In the process of blending the coelom becomes continuous 

 across the mid-line by the apparent loss of the medial margin of 

 each lateral plate ; the medial margins together constitute the dorsal 

 mesocardium which is, thus, early lost. The entire coelom, paired 

 or unpaired, occurring in Fig. 7 is pericardial. The site of discharge 

 of the jugular veins (which later determines the points of separation, 

 on either side of the embryo, between the pericardial and peritoneal 

 regions of the original coelom) will occur slightly behind the site 

 of Fig. 7G. 



The endocardium has undergone very rapid growth backwards (see 

 Fig. 7G), and has now reached the anterior limit of the first body 

 somite. The interval l^etween the first body somite and the head is 

 some little distance behind (caudad from) the posterior limit of the 

 reconstruction and the future site of discharge of the jugular veins. 

 The ventral surface of the entodermal pharynx and of the adjacent 

 region of the (peritoneal) splanchnic mesoderm is, therefore, in 

 the head region posterior to the reconstruction, covered ventrally 

 by the endocardium. As far as has been ascertained, the endocardium 

 does not encroach upon the region ventral to the first body somite it- 

 self ; a re-investigation of this difficult point will form a part of a 

 study of the origin of the body-vessels to be undertaken at a later 

 date. 



The descent area of the portion moyenne has moved slightly 

 backwards since the preceding stage ; it has narrowed considerably 

 (in the antero-posterior dimension), and now consists of only a 

 narrow cord of cells on each side. The term "descent area" which 

 has hitherto been used to designate cellular connection between the 

 endocardium ventral to the lateral plates and the portion moyenne 

 dorsal to them is no longer applicable, for descent has ceased; the 

 cells between the entodermal pharynx and lateral plate (seen on each 

 side in Fig. 7D) represent the first (transverse) part of the ventral 

 aorta, and the portion moyenne, as such, has ceased to exist. 



