82 



ARACHNIDA. 



The enteron in the Araneae arises from the entoderm-cells which 

 are distributed in the yolk. It begins to form at the posterior 

 end of the body, and in this respect there is a resemblance to the 

 Scorpiones ; but a similar rudiment soon (or perhaps simultaneously) 

 appears in the anterior part of the body (Fig. 41 A and B, md). 



At a late stage of development, only a few days before hatching, 

 there appears at the anterior end of the proctodaeum an accumulation 

 of entoderm-cells (Fig. 41 A, md), which soon become arranged 

 regularly at the periphery of the yolk to form an epithelium. There 

 thus arises a trumpet-like structure, closed posteriorly and open 



anteriorly, which is 

 m. hr the posterior part of 



the rudiment of the 

 enteron (Fig. 41 B, 

 md). A structure in 

 all respects similar 

 appears anteriorly at 

 the blind end of the 

 stomodaeum {md). 

 This also arises from 

 the yolk-cells, which 

 have greatly increased 

 in number. The 

 enteron is completed 

 when the two parts, 

 which grow out 

 towards each other 

 with their wide-open 

 ends, finally unite. 

 The stomodaeum and the proctodaeum then fuse with the enteron. 

 But before this happens another more complicated structure, the 

 liver, makes its appearance. Even before the development of the 

 enteron began, a considerable number of folds (Fig. 42, /) appeared 

 in the splanchnic layer of the mesoderm, which (in the absence of 

 the entodermal epithelium) lay directly on the yolk. These folds 

 grow into the mass of yolk, almost completely isolating certain 

 complexes from the principal mass (Fig. 41, /). In these isolated 

 yolk-complexes the formation of the epithelium is said to take place 

 later in the same way as in the main mass itself, viz., by the yolk- 

 cells coming to the surface, and becoming arranged in a regular 

 layer (?) The epithelium thus formed now lies close under the 



Fig. 42. — Transverse section through the abdomen of an 

 embryo of Pholcus phalangioides (after Morin). dz, yolk- 

 cells ; /, folds of the splanchnic layer ; h, heart ; in, muscles ; 

 so, somatic, sp, splanchnic layer of the mesoderm. 



