464 F. W. GAMBLE AND J. H. ASHWORTH. 



peared in examples measuring 65 mm. in length. At this stage 

 of growth, the wall of the heart consists of an outer peritoneal 

 cubical epithelium and an inner but indistinct endothelium. 

 Between these two layers it is not possible to detect any mus- 

 cular tissue. The cavity of the heart is, however, invaded by 

 strands of cells which repeat the structure of the heart wall, 

 and are probably invagiuations of it. In A. Grubii the in- 

 vagination is clearly marked (figs. 38, 39). Later on, as the 

 muscular tissue develops in the wall of the heart, fresh 

 invaginations occur, composed of an extremely delicate en- 

 dothelium, a muscular layer, and a mass of cells, some 

 granular, some glandular, forming a fairly definite lining to 

 the invagination, but projecting at their free ends into an 

 irregular lumen (PI. 25, figs. 41 — 43), partially blocked up 

 by cells within which yellowish or yellowish-brown granules 

 may be seen. The cells cannot, however, be said to form a 

 medullary layer. In some places the granules are larger, 

 and united into a spherical mass lying in a vacuole ; in 

 others very minute and scattered (fig. 43). They agree 

 in appearance with the chlorogogen granules of the peri- 

 toneum. 



In A. Grubii, 140 mm. long, the first traces of the heart- 

 body are found as a few short and apparently solid ingrowths 

 of the muscular and peritoneal layers of the heart wall (fig. 

 38). These ingrowths occur on the posterior (and to some 

 extent outer) surface of the heart, where the muscular layer is 

 specially developed ; elsewhere it is indistinct. The granular 

 chlorogogenous bodies are in this way carried into the cavity 

 of the heart. A distinct endothelium forming the outer wall 

 of the process can in some cases be detected with certainty 

 (fig. 41). Within this there was a muscular layer, while the 

 centre of the process is formed by somewhat cubical peri- 

 toneal cells arranged in a netwoi'k rather than as a distinct 

 cortex, and between them muscular fibres are occasionally 

 seen (figs. 41 — 43). The outlines of the more centrally 

 placed cells are somewhat difiicult to make out. Within 

 these cells fine yellowish-brown granules occur, the larger 



