86 



ARACHNIDA. 



segments occurs last, the two niesoderm-bands are connected. 

 Differentiation takes place from before backward, except in the 

 most anterior segments, which, as already mentioned, in the Araneae 

 as well as in the Scorpiones, become distinct somewhat later than 

 the following cephalo-thoracic segments. The number of the primi- 

 tive segments corresponds to that of the body-segments, one pair 

 of the former occurring in each of the latter. The cephalic lobes 

 also contain pairs of primitive segments, as the descriptions of 

 Balfour, Morin, and Kishinouye undoubtedly show. Here, again, 

 we have a resemblance to the Scorpiones (Fig. 13 A, p. 22). In 



Fig. 45. — Transverse sections through embryos of Theridium macvJatum (after Morin). In A, 

 the embryo, which is curved round the yolk, is cut through twice ; the thoracic limbs and 

 primitive segments can be recognised below, while the abdominal primitive segments are 

 seen above. B, cross-section through the abdomen of an older embryo, in which the primi- 

 tive segments have increased in size, bl, blood-corpuscles ; d, yolk ; dz, yolk-cells ; ex, 

 limbs; /, lung invaginations; n, rudiment of the chain of ganglia; us, primitive 

 segments. 



the cephalo-thorax and also in the abdomen, as far as the latter 

 possesses appendages, the primitive segments extend into the limbs ; 

 indeed for the time they withdraw almost entirely into the limbs 

 (Figs. 44 and 45 A). The mesoderm-bands naturally also take part 

 in the displacement undergone by the two halves of the germ-band 

 in consequence of the pressing forward of the yolk-mass to the 

 ventral side. Whereas they formerly lay near the ventral median 

 line (Fig. 43 C) the}' now appear removed from it, and divided by 

 the so-called yolk-sac (Fig. 29 A, p. 53), 



The segmental cavities 



