THE BLOOD-VASCULAR SYSTEM AND THE BODY-CAVITY. 91 



merits) gradually disintegrate, the cells falling into the body-cavity 

 (Schimkewitsch). These cells probably give rise to blood-corpuscles. 

 The primitive segments of the cephalic lobes seem already to have 

 fused with those of the cheliceral segment, at least Schimkewitsch 

 speaks of a connection between the two which, however, he explains 

 in another way. 



If we understand Schimkewitsch rightly, he assumes that the pair of 

 primitive segments in the cheliceral segment arise by division from the pair 

 in the head ; we should lie more inclined to assume the opposite of this, i.e., 

 an extension of the first trunk-segment into the cephalic region. It, however, 

 appears from the accounts and figures before us that the cephalic and cheliceral 

 segments undoubtedly have separate primitive segments. A union of these two 

 pairs of segments, like that described by Kleinexbehg for Lumbricus, would 

 then take place. 



The two segmental cavities of the head become united ; such a 

 union of the cavities of the two sides must take place in the trunk 

 also as a result of the processes described in connection with the 

 formation of the heart (Fig. 47). This at least applies to the dorsal 

 side ; on the ventral side, the primitive segments are at first still far 

 apart (Fig. 46), but they shift gradually towards the middle line, so 

 that they finally extend round the whole mass of yolk. In the 

 abdomen the primitive segments remain separate longer, a fact 

 which is in keeping with their later differentiation. Even when 

 they are fused together, the mesoderm represents two extensive 

 layers passing into one another — an outer or somatic layer and an 

 inner or splanchnic layer ; between these is the secondary body- 

 cavity (Schimkewitsch). 



From the splanchnic layer, the folds already mentioned in con- 

 nection with the formation of the intestine grow into the yolk 

 (Fig. 42, p. 82), in this way cutting off from it isolated masses 

 which correspond to the later hepatic lobes. "We should like here 

 to draw special attention to the important fact that the yolk is so 

 long a time bounded solely by mesoderm (Figs. 46 and 47), and 

 that the epithelium of the enteron develops very late (Fig. 41, p. 80): 

 indeed, the mapping out of a large part of the enteron, that of the 

 liver, seems to be commenced by the mesoderm. 



Whether the distribution of these folds corresponds to a true segmentation 

 appears doubtful, although this might be indicated by the appearance of four 

 lateral folds in the cephalo-thorax. It appears that these correspond to the 

 thoracic caeca of the enteron (?), for in the abdomen also a number of folds 

 occur, and it is these principally that give rise to the form of the liver (Morin). 

 The folds which penetrate the yolk not only come from the side, but from 

 above and below, and thus represent oblique as well as longitudinal 1 1\ ■ i - 



