DEVELOPMENT OF THE BLOOD VESSELS IN THE EROG. 191 



at the sides of the pharynx, but beneath the middle part of its floor 

 is separated from it by a considerable interval, so that beneath the 

 floor of the hinder part of the pharynx (c/. Figs. 2 and 3) there is a 

 space, somewhat quadrangular in transverse section, bounded above 

 by hypoblast, and below and at the sides by the splanchnic layer of 

 the mesoblast. 



Within this space lies a tube with thin walls, H, which is free 

 along the greater part of its length, but attached in front to the 

 hypoblast of the floor of the pharynx (Fig. 2, H), and continuous 

 behind with the anterior surface of the liver rudiment. This tube 

 forms the endothelial lining of the heart. It consists of a single 

 layer of cells, somewhat variable in shape and size, but the majority 

 of which are slightly flattened in shape, and in contact with their neigh- 

 bours at their edges. At the hinder end of the endothelial tube the 

 cells are more spherical and more richly laden with yolk granules 

 than those of the anterior part ; and a few of these spherical cells 

 are commonly seen lying within the cavity of the tube (Figs. 2 and 

 3, H). The tube, as seen in Fig. 2, H, is closed in front by a 

 wall of its own, distinct from the hypoblast of the floor of the 

 pharynx ; at the hinder end the tube has no endothelial wall, but 

 is closed by the anterior surface of the liver (Fig. 2). 



The muscular wall of the heart is formed from the splanchnic 

 mesoblast (Figs. 2 and 3, H B), which, at the present stage, 

 consists of a single layer of short columnar cells surrounding the 

 endothelial tube, H, below and at the sides. Later on, this meso- 

 blast grows in from either side towards the middle line between , 

 the floor of the pharynx and the endothelial tube. In this way the 

 mesoblastic investment of the heart, from which the muscles are 

 formed, is completed, as is also the mesoblastic wall of the pharynx. 

 For a time these two structures remain connected by a median 

 vertical mesocardial fold, formed by the meeting of the mesoblastic 

 ingrowths from the two sides ; later on, this mesocardial fold is 

 absorbed. 



So far the account we have given of the formation of the heart is 

 in harmony with the descriptions of all investigators. Concerning 

 the earlier stages of development, there is, however, great difference . 

 of opinion, more particularly with reference to the origin of the 

 colls which form the endothelial lining of the heart. 



