VIII 



ARTHROPODA 



235 



Its innermost portion enlarges to form the so-called stomach, whilst 

 its outer portion forms the pharynx and has its cuticular lining 

 produced into parallel ridges. The oesophagus is the narrow portion 

 connecting the pharynx and stomach. At the hinder end of the 

 stomach an accumulation of endoderrn cells is found abutting on 

 the yolk, which now becomes indented by the ingrowth of four pairs 

 of mesodermic septa, and the yolk lobes thus outlined eventually form 

 the lobes of the liver. These septa seem to be the outgrowths 

 from the abdominal coelornic sacs, whose cavities have disappeared. 



Kautsch maintains 

 that the epithelium cover- 

 ing the lobes of the liver 

 is derived from the meso- 

 dermic cells of these septa. 

 This is excessively un- 

 likely, and Kishinouye's 

 statement that it is derived 

 from the anterior accumu- 

 lation of "yolk cells" 

 situated where the endo- 

 derm abuts on the yolk, 

 and which, according to 

 him, gradually spread 

 throughout it and rise to 

 the surface, is infinitely 

 more likely. In Astacus 

 we have already learnt 

 that the liver lobes arise as 

 a result of the indentation 

 of the yolky endodermic 

 sac by mesodermic septa, 

 but in that animal there 

 is no question that the 

 epithelium of the liver 

 arises from the endodermic 

 nuclei and their surround- 

 ing cytoplasm, and this is also probably the case in the spider. The 

 appearance of the lumen of the mid-gut first in the region of the 

 stercoral pocket is paralleled in Astacus by the first appearance of 

 ..Definite " endodermal plate " in the hinder region of the mid-gut. 



The development of the embryo is now complete and it is 

 hatched.- 



ster 



FIG. 186. Horizontal section through abdomen of an 

 advanced embryo of Agelena labyrinthica to show 

 the division of the yolk into lobes by septa. (After 

 Kautsch. ) 



H, heart ; m.g, hinder part of mid-gut ; sep, mesodermic 

 septa dividing the yolk ; ster, stercoral pocket. 



OTHER AEACHNIDA 



What is known of the development of other Arachnida demon- 

 strates that it is in remarkable agreement with that of the spider, 

 in all essentials, but it is only in the case of Limulus and the 



