The Dcvclopinciit of the Heart in Shad. 233 



evident; mention of these, since they had no direct bearing on the 

 subject in liand, has, hitherto, been omitted. In the following notes 

 the facts alluded to are briefly reviewed, and an attempt made to 

 estimate their significance. 



The portion of the head shown in Fig. 7 may be divided trans- 

 versely into three regions : anterior, middle and posterior ; of these 

 the middle region extends from the back of the red circle, indicating 

 the aortic root, back to the place at which the entodermal pharynx 

 becomes nan*ow behind the hyo-branchial anlage ; the anterior region 

 corresponds to the pericardial area in front of the middle region, 

 and the posterior to the corresponding area behind it. 



In the embryo of 15 somites, from which sections are shown in 

 Fig, 3, the cells immediately adjacent to the medial borders of the 

 lateral plates are isolated from the remainder of the mesoderm to 

 form the portion moyenne of Swaen and Brachet ; this isolation of 

 the portion moyenne occurs only in the anterior and middle regions 

 (comparing the stages of 15 and 32 somites approximately) and not 

 in the posterior. The endocardium, as seen in Fig. 7, is derived 

 exclusively from the portion moyenne of the anterior region. Having 

 recalled these points regarding the portion moyenne and endocardium, 

 the occurrences bearing on the vascular endothelium belonging to 

 the three regions mentioned will, as far as possible, be considered 

 separately. 



The anterior region may be examined first. In Fig. YD the lateral, 

 now pericardial, plate is not in contact with the ventral surface of 

 the entodermal pharynx, as is the case elsewhere (save in the two 

 neighboring sections), contact being prevented by a cord of cells 

 on either side. The cells of these cords were derived from the por- 

 tion moyenne, and, like other cells known to come from the same 

 source, they are at this stage easily distinguishable from the ordinary 

 mesoderm. 



Fig. 9 is from a reconstruction of the aorta at the earliest stage 

 in which it appears as a well defined vessel throughout the head 

 (42 somites; the ventral surface of this reconstruction is showm as 

 Fig. 12). The cells between the entodermal pharynx and pericardial 

 plate in Fig. YD correspond exactly in position with the first (or 

 transverse) part of the ventral aorta. 



