422 GENERAL CONSIDERATIONS ON 



the proctodaeum. We are therefore far more inclined to regard the Malpighian 

 vessels as new structures coming into use on the degeneration of the nephridia, 

 than to trace them back to the actual nephridia. 



The development of the enteron is essentially influenced by the 

 relations of the entoderm-rucliment to the mass of the food-yolk. 

 The latter, which originally fills the cleavage-cavity, is taken up 

 later into the enteron. This process may take place in various 

 ways: (1) the yolk may filter through the wall of the enteric sac 

 (Astacus), or (2) the entoderm-cells may wander through the yolk to 

 constitute the enteric epithelium at its surface (Crustacea, Limulus, 

 Araneae, Chilopoda), or finally (3) the food-yolk may be grown 

 round by the entoderm -rudiment (Mt/sis, Isopoda, Scorpiones (1) 

 Insecta). The formation of the intestinal epithelium in some cases 

 only takes place very late (Araneae), and then the splanchnic layer 

 of the mesoderm, which has meanwhile developed, becomes closely 

 applied to the yolk-mass. Septa-like processes then grow out from 

 it into the yolk-mass and isolate single complexes of the latter, 

 which appear like diverticula. As in the central part of the enteron, 

 so also in these diverticula, the formation of the epithelium only 

 commences at a later period. The diverticula represent the rudi- 

 ments of the hepatic lobes, which, in the Crustacea, are formed in 

 the same way, the only distinction being that, in the latter, the 

 differentiation of the epithelium takes place much earlier. 



Since, in some Arthropoda, only a part of the food-yolk is taken 

 up into the interior of the intestine, it may happen that smaller or 

 greater masses of yolk remain behind in the body-cavity and there 

 undergo a gradual absorption (Moina, Mysis, and the Dipterous 

 Insects). In the Diplopoda, this condition, which elsewhere is 

 exceptional, appears developed to a high degree, for here the enteron 

 is said to arise as a somewhat narrow tube in the middle of the 

 yolk-mass. As a consequence of this, the greater part of the yolk 

 would come to lie in the body-cavity. Here, as in the Crustacea 

 above mentioned, the yolk-mass in the body-cavity is thickly sur- 

 rounded and interpenetrated by mesoderm-cells. 



We must next consider the development of the mesoderm. The 

 coelomic cavities of the primitive segments, which, in some cases 

 (Perijxxtus and the Arachnida), have been seen to attain such high 

 development, do not, in the Arthropoda, become the definitive 

 body-cavity, but sooner or later the primitive segments undergo 

 degeneration. But before this occurs, the formation of the heart 

 starts from the primitive segments, single cells of the coelomic sacs 



