FORMATION OF THE MESODERM. 353 



the oral aperture above which projects an " upper lip." The stomo- 

 daeum runs upward and forward, becomes dilated (anterior stomach), 

 and then bends sharply backwards to join the enteron. Its strongly 

 cuticularised inner wall is longitudinally folded. The anal aperture 

 lies immediately in front of the insertion of the caudal spine ; the 

 proctodaeum is short throughout life. 



The enteron here attains its final form unusually late, a peculiarity 

 which recurs in the Scorpiones. During the whole of embryonic 

 life the rudiment of the enteron consists of the mass of food-yolk 

 (Fig. 163, d) which has undergone cleavage, and is divided up into 

 polygonal cells, and the surface of which in later stages appears 

 covered with a splanchnic layer arising from the mesoderm. At first 

 this mass of entoderm retains the spherical shape, but it adapts 

 itself later to the form of the embryo. Its anterior region, however, 

 which lies in the cephalo-thorax, very soon becomes divided up into 

 lobes by mesodermal septa, which grow in laterally ; these lobes are 

 the first rudiments of the digestive gland. There are, at first, six of 

 these primary lobes on each side (p. 346), but these soon present a 

 branched appearance by the formation of secondary lobes (Figs. 160 

 and 161).* In consequence of the development of these mesodermal 

 septa and of paired hepatic lobes, the organs lying within the cephalo- 

 thorax show a segmentation corresponding to the six thoracic segments. 



The transformation of the solid entoderm into the hollow enteron 

 takes place by the increase in number of the yolk-cells near the 

 surface, and by their arrangement as a single-layered epithelium, 

 which very soon separates from the central mass of yolk, liquefied 

 yolk-substance collecting between the latter and the epithelium. 

 The stomodaeum breaks through into the enteron earlier than does 

 the proctodaeum. The hepatic lobes become grouped about two 

 pairs of efferent ducts, which open into the anterior part of the met- 

 enteron. Although the greater number of the hepatic lobes belong 

 to the cephalo-thorax, one pair of hepatic tubes which opens into 

 the second pair of efferentjducts extends back into the abdomen. 



C. Formation of the Mesoderm. 



The mesoderm arises, as we have already seen (p. 342), as a pro- 

 liferation of cells proceeding from the primitive groove and spreading 

 out beneath the ectoderm. When the rudiments of the limbs 

 appear, the mesoderm splits along the middle line, so that it now 

 consists of two mesodermal bands running above the attachment of 



* On the development of the mesodermal septa, cf. p. 354. 

 2 A 



