HEART. 177 



hepatic tubes, which become connected later on the ventral side by entodermal 

 epithelium. Two longitudinal folds arise and fuse with the inwardly curving 

 edges of the grooves, and thus separate the hepatic tubes from the middle 

 portion of the intestinal canal. At the same time, the hepatic tubes become 

 divided through longitudinal folding into four secondary tubes in the same 

 way as in the Arthrostraca. It appears that, in Mysis, when the mid-gut 

 vesicle forms, the whole of the food-yolk is not taken up into it, but part of 

 it comes to lie in the cephalic region outside of the intestine, and thus in 

 the body-cavity. The food-yolk has a similar position in Moina. 



G. Heart. 



' In describing the development of the heart, we must take as our 

 starting point the observations made by Claus (Nos. 8 and 9) on 

 Branchipus. The somatic layer of the mesoderm here forms a cell- 

 stratum originating on the ventral surface, and now divided into 

 separate segments; this mesoblast gradually grows upwards under 

 the lateral parts of the integument. The dorsal edge of these 

 .growing mesoderm-segments is formed by a single row of large 

 cells (cardioblasts, Nusbaum, Figs. 87, c, and 88 A, c), which later 

 assume a crescentic shape (Fig. 88 B, c), so that a channel is now 

 formed on each side. These semilunar channels, meeting and fusing 

 in the middle line, give rise to a dorsal tube (Fig. 88 C, c). The 

 latter is from the very first divided up into separate segments 

 (chambers) corresponding with the primitive mesoderm-segments ; 

 the boundaries of these chambers persist as the lateral ostia. This 

 origin of the heart seems distinctly to show that its lumen must 

 be considered as a remains of the primary cleavage-cavity (Butschli, 

 Schimkewitsch). 



The ostia develop at the boundaries of the mesoderm-segments. The cardio- 

 blasts become transformed into the muscle-cells of the wall of the heart. 

 During their development, the latter have their lower ends connected with the 

 dorsal portion of the intestinal muscle-layer. From this point, a horizontal 

 septum stretches out towards the body-wall ; this is the pericardial diaphragm 

 (Fig. 88 C, s), which separates an upper portion of the coelom containing the 

 heart from the rest of the body-cavity. This septum is found in all Crustacea. 



The heart of Oniscus develops in a similar way by the fusing of two grooves 

 which arise from a single row of cells on each side (Nusbaum). The formation 

 of the heart in the Amphipoda (Pereyaslawzewa, Kossijskaya) also passes 

 through a similar stage. While, however, in Oniscus, the posterior parts of the 

 l>o<ly develop first, and the formation of the heart proceeds gradually from 

 behind forward, in the Amphipoda, the heart begins to form in the middle 

 region of the body. A vascular trunk arises simultaneously in front of the 

 dorsal organ in the same way. The two rudiments only fuse after the dorsal 

 organ has degenerated, that organ retarding the development of the single 

 •dorsal tube. In the Amphipoda the fusing of the two grooves takes place in 



N 



