374 



ARACHNID A. 



of the ventral part of the mesoblast. At the lines of junction of 

 successive somites the splanchnic layer of mesoblast dips into the 



yolk, and forms a 

 number of transverse 

 septa, which do not 

 reach the middle of 

 the yolk, but leave a 

 central part free, in 

 which the mesente- 

 ron is subsequently 

 formed. At the in- 

 sertion of these septa 



?mf^\m^i W" there are developed 



L^fc ^a/Mi widish spaces be- 



tween the layers of 

 somatic and splanch- 

 nic mesoblast, which 

 form transversely di- 

 rected channels pass- 

 ing from the heart 

 outwards. They are 

 probably venous. At 

 a later stage the septa 

 send out lateral off- 

 shoots, and divide the 

 peripheral part of the 

 abdominal cavity into 

 a number of compartments filled with yolk. It is probable that the 

 hepatic diverticula are eventually formed in these compartments. 



The somatic layer of mesoblast is converted into the muscles, 

 both of the limbs and trunk, the superficial connective tissue, ner- 

 vous sheath, etc. It probably also gives rise to the three muscles 

 attached to the suctorial apparatus of the oesophagus. 



The heart and aorta are formed as a solid rod of cells of the dorsal 

 lue.soblast, before it is distinctly divided into splanchnic and somatic 

 layers. Eventually the central cells of the heart become blood cor- 

 puscles, while its walls are constituted of an outer muscular and inner 

 epithelioid layer. It becomes functional, and acquires its valves, 

 arterial branches, etc., by the stage represented in fig. 201 B. 



The history of the mesoblast, more especially of the mesoblastic somites, 

 of the Scorpion is very similar to that in Spiders : their cavity is continued 

 in the same way into the limbs. The general character of the somites 

 in the tail is shewn in fig. 207. The caudal aorta is stated by Metschnikoff 

 to be formed from part of the mesenteron, but this is too improbable to be 

 accepted without further confirmation. 



The hypoblast and alimentary tract. It has already been stated 

 that the yolk is to be regarded as corresponding to the hypoblast ot 

 other types. 



FIG. 206. TRANSVERSE SECTION THROUGH THE THORA- 

 ( 1C REGION OF AN EMBRYO OF AGELENA LABYRINTHICA. 



The section is taken from an embryo of the same age 

 as fig. 201 A, and passes through the maximum pro- 

 tuberance of the ventral yolk sack. 



vn. ventral nerve cord; yk. yolk; me.s. mesoblastic 

 somite ; ao. aorta. 



