20 ARACHNIDA. 



enteron first develops fully, arising from this cell-mass in the form of 

 a tube which, like this body-region itself, is at present neither thick 

 nor long. The gut continues to develop from this region, the 

 epithelium differentiating first on the ventral surface, and gradually 

 extending dorsally. The whole of the yolk was surrounded by 

 entoderm at an earlier stage, but it appears that the cells of the 

 entoderm resemble those we have already met with in the Crustacea 

 (Vol. ii., p. 129). They are greatly swollen and are of a cylindrical 

 shape, so that they do not yet resemble the future intestinal 

 epithelium ; they are chiefly occupied in the assimilation of the 

 yolk. This provisional epithelium gradually changes into the 

 definite intestinal epithelium from behind forward, first developing 

 ventrally, and then extending towards the dorsal side. The hepatic 

 caeca, which at first contain abundant masses of food-yolk, form as 

 outgrowths of the provisional epithelium, perhaps also caused by 

 the inward pressure of the folds of the splanchnic layer of the 

 mesoderm, as in the Araneae (p. 82). It appears as if the hepatic 

 tubes were segmentally arranged. By further extension and rami- 

 fication, the liver attains its definite form. 



It seems probable that the above description of the development of the 

 enteron is correct, since Laurie, as veil as Kowalevsky and Schulgin, speak 

 of a somewhat early and complete circumcrescence of the yolk by the entoderm. 

 We have not, however, succeeded in obtaining a perfectly clear idea of these 

 processes from the works of these observers. They might be understood to- 

 imply that the circumcrescence just mentioned is only complete at some parts, 

 e.g. anteriorly, where the germ-band grows round the pole of the egg, and 

 especially posteriorly, and that the circumcrescence of the yolk by the entoderm 

 only takes place from behind (progressing ventro-dorsally), the entoderm soon 

 developing into the definitive intestinal epithelium. This method of origin of 

 the intestine would somewhat resemble that in the Araneae, where the epithelium 

 of the enteron grows round the yolk-mass, which is directly bounded by mesoderm 

 (p. 82). We are not, however, able to obtain this idea from the treatises under 

 review, and there would even then be considerable difference between this 

 process and that in the Araneae, since, in the latter, the first rudiment of the 

 enteron is said to form from an accumulation of yolk-cells. 



The two long, tubular intestinal appendages which until now have 

 been called Malpighian vessels, and thus regarded as homologous with 

 the synonymous organs of the Insects and Myriopoda, arise in the 

 last segment of the pre-abdomen as outgrowths of the enteron 

 (Laurie). The two outgrowths form comparatively far forward on 

 the enteron at a time when the proctodaeal invagination has not yet 

 appeared. When this arises, the Malpighian vessels are seen opening 

 into the intestine in the pre-abdomen ; their point of origin is thus 



