THE FURTHER DEVELOPMENT OF THE EMBRYO. 



147 



with yolk, grow out at an early stage into the limbs (di). These are 

 the intestinal caeca, which, in the larva (Fig. 72 A), as well as in 

 the adult, run far into the limbs. This arrangement recalls that in 

 Chelifer, where the yolk also extends far into the limbs (p. 29, and 

 Fig. 16). This is also the case in the Acarina, and in the embryos 

 of some Araneae, e.g., Agalena (Locy). This peculiar feature is 

 known to be retained throughout' life in the Pantopoda, in which 

 the trunk is much reduced as compared with the limbs. These 

 latter also contain the genital organs in the adult, and this explains 

 the fact that a process of the mesoderm at an early stage runs 

 between the ectoderm and the entoderm into the rudiment of the 



mis* 



Fig. 68.— Transverse section through an embryo of Pallcne empusa at a somewhat older stage 

 than in Fig. 07 A. The ventral depressions (e) have closed (after Morgan), bg, ventral 

 nerve-Btrand showing fibrous structure on the dorsal side ; eoe, mesodermal cavity in the 

 limb ; rf, yolk ; di, intestinal caeca of the limbs ; e, the closed ectodermal invagination ; 

 ect, ectoderm ; ent, entoderm ; mes, mesoderm ; }\ pair of ambulatory limbs. 



limb. According to Morgan, a cavity bordered by a mesodermal 

 •epithelium lies at the base of each limb, the mesodermal process 

 •extending from this point into the limb (Fig. 68, mes). Morgan 

 ■does not hesitate to speak of the body-cavity of the limbs. In any 

 case we thus have here the primitive segments which, taken together, 

 represent the two already segmented mesoderm-bands. These latter, 

 together with the rudiments of the ganglionic chain and the limbs 

 on each side, form the germ-band (Fig. 66), although this is con- 

 siderably reduced in accordance with the small size of the egg. As 

 these mesoderm-bands develop at the thickened part of the blastoderm, 



