WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 85 



that their appearance and arrangement may be altered in a variety of ways by 

 reagents or post-mortem changes. The extent and importance of these intercellular 

 and interepithelial connections of early development (the eytodesmata) may be 

 seen from the detailed accounts of Szily (1904, 1908) and Held (1909). Taken en 

 masse, as they are found throughout the embryo, they make up the mesostroma 

 of Studnicka (1911, 1912). Aside from their obvious office of retaining the early 

 cell masses and layers in proper order and arrangement (Szily), they would also 

 afford adequate means for the necessary diffusion of fluids at this stage, while later 

 they may play an important role in connection with developing nerve paths (Held). 

 The embryonic heart offers an example of the special development of the 

 eytodesmata just mentioned. In the particular case in question their arrangement 

 is essentially radial, extending from the future endothelial tube within to the under 

 surface of the mantle externally, and also, but to a slightly less extent, to the ventral 

 wall of the fore-gut behind. As a rule, the course described by the fibrils is rather 

 wavy and irregular, often simulating a loose, much drawn out reticulum, and they 

 show a distinct tendency to clump together, especially internally, where they spring 

 from the various prominences and ridges of the plexus. That they exert any trac- 

 tion upon the nascent endothelium, or later upon the heart tube, drawing them out 

 at divers points as has been suggested, is altogether possible, but appears to us 

 unlikely. Often between the fibrils there occur regular, open spaces, possibly to 

 some extent of the nature of artefacts, and where there are cells in close relation 

 with them they may simulate angiocysts. Identical conditions are figured by Szily 

 in the chick heart (1. c, 1908, figs. 5 and 8). A large space of this kind is found quite 

 constantly in the dorsal concavity of the plexus, between it and the fore-gut. The 

 great development of such a protoplasmic, intercellular network between the 

 myocardial and endocardial layers of the heart wall is doubtless to be looked upon 

 as a preparatory feature, facilitating or initiating the later invasion of the space 

 thus occupied by the ventricular myocardium on the one hand and, on the other, 

 by the endothelium at various points in the formation of the endocardial cushions 

 and other connective tissue of endothelial origin. Toward the anterior limits of the 

 myoendocardial space the characteristic arrangement of the fibrils is gradually 

 obscured; they become more irregular, less numerous, and the whole picture is 

 that of a coarser, less orderly framework, until it finally becomes indistinguishable 

 from the ordinary mesostroma of the neighboring somatopleure. The arrangement 

 of the reticulum in the heart under consideration is quite different from that shown 

 by Szily (1. c, 1904) in the heart of chick embryos, where the network is looser and 

 the fibrils are arranged without any special regularity, no predominently radial 

 disposition being in evidence. Similar differences are obvious in the section through 

 the heart of a 15-somite human embryo as given by Tandler (1912). Here, although 

 it is of a much later stage, the network, as in the chick, has about as indefinite an 

 arrangement as could be desired. The nearest approach to the conditions present 

 in our case are those shown by the former writer, but taken from the bulbus cordis 

 of a 32-hour chick embryo (1. c, 1908, figs. 5 and 6). 



The inner basal surface of the mantle, to which are attached the fibrils, is 

 remarkably smooth and even, more so in fact than the same surface on any of the 



