132 MALCOLM LADETE. 



intestine. They contain^ however, a rather larger proportion of 

 yolk. The scattered layer of hypoblast cells, which in the 

 preceding stages surrounded the yolk, takes a large part in their 

 formation. They open into the intestine in pairs by wide 

 ducts. 



The Malpighian tubes (PL XVIII, fig. 60, mlph.) have not 

 undergone much development. They reach well forward in the 

 body, and open into the intestine in the first caudal segment. 



It is evident from the structure of the intestine that the 

 young scorpion does not need food for some time after hatch- 

 ing. The large amount of yolk which still exists must last it 

 for some weeks, or most probably till the next spring. If this 

 is the case embryonic life practically lasts twelve months as the 

 eggs are fertilized in May. 



The outer layer of the mesoblast has now for the most part 

 formed itself into muscles. The inner layer is very much com- 

 plicated, being folded in so as to surround the gut and the 

 lobes of the liver. The spaces between the lobes of the liver, 

 which are undoubtedly the true coelom, are filled up by a net- 

 work of trabecular tissue (PI. XVIII, fig. 60). The heart, 

 pericardium, and blood-vessels are fully formed and contain a 

 considerable number of large nucleated corpuscles. 



Summary of the Changes during the Fourth Period. 



(1) The thoracic appendages begin as simple outpushings of 

 the body wall containing a portion of the coelom (Stage I). 



They rapidly increase in length and the chelicerse and chelte 

 become bifid at their extremities. Sternocoxal processes are 

 present on the third to sixth appendages (Stage K). 



The chelicerse, which were at first behind the mouth, gradu- 

 ally move forward relatively to it till they come to lie in front 

 of it (Stage L). 



(2) The coxal gland begins as a simple tube opening to the 

 outside at the base of the fifth pair of appendages, and opening 

 at the other end into the coelom (Stage K). The tube soon 

 becomes coiled, but the external opening persists until after 

 hatching. It is undoubtely a uephridium. 



