24 



OUTLINE OF THE ARACHNID THEORY. 



locomotor appendages and their voluminous muscles disappear, and also numerous 

 endocranial muscles, owing to the union of the endocranium with the dermal 

 skeleton. Finally the branchial appendages lose a part of their muscles in their 

 conversion into lung-book-like gill pouches. 



This progressive degeneration of the cephalic mesoderm, from before back- 

 ward, has been, therefore, an ever present factor, exercising a persistent and power- 

 ful influence over the form of the head and the structure of the brain thoroughout 

 the whole arthropod-vertebrate phylum. 



The Vascular Area and Concrescence. 



Vascular Area.— The mode of growth of the extra embryonic area, the 

 concrescence of the germ wall, and the character of the mesoderm in the various 

 regions is shown in Fig. 138. 



Fig. 17. — A-C , Scorpion embryos in side view, semi-diagrammatic. The thoracic appendages are removed in B 

 and C. D, Diagram indicating relations of the cephalic organs in arachnids to those in vertebrates. 



The margin of the germinal area belonging to the thoracic metameres is 

 greatly thickened, forming large masses of spherical or oval cells containing a 

 small excentric nucleus, and a brilliantly refractive, colorless thread, usually 

 coiled with great regularity in the long axis of the cell. (Fig. 131.) 



Some of these cells are ultimately converted into muscles, others remain as 

 free amoeboid cells, and in the adult may be found in great numbers scattered 

 among the connective tissue lacunae, in the anterior part of the cephalothorax. 



Whether the degenerating muscle cells of the cephalothorax are to be re- 

 garded as true blood corpuscles or not is doubtful; but it is evident that owing 

 to the increase in size of the yolk sphere, there is already established, in the higher 



