BLOOD-VASCULAR SYSTEM AND COELOM. 23 



Their extension towards the dorsal surface is, however, specially 

 noticeable. While this dorsal extension is less marked in the 

 anterior segments, it is very striking in the abdomen, where the 

 primitive segments soon grow beyond the area of the germ-band 

 towards the dorsal side. This is well marked in ■ the posterior 

 region in Fig. 7 B, though these segments are shown somewhat too 

 distinct. As the anterior segments undergo the same process, the 

 whole of the mesoderm, pressing forward between the ectoderm and 

 the entoderm, extends dorsally. The outer wall of the primitive 

 segments (the somatic layer), is now thicker, being composed of 

 several layers of cells, while the inner wall (the splanchnic layer) 

 consists of a single layer of cells. The pair of primitive segments 

 in the cephalic region has specially thin walls, the lumen also being 

 comparatively small (Laurie). 



The extension of the mesoderm dorsally is not caused by the 

 mere enlargement of the primitive segments with their cavities, but 

 this extension progresses in such a way that, dorsally, where the 

 somatic and splanchnic layers unite, the common rudiment continues 

 to grow upwards as a single layer of cells (Kowalevsky and Schulgin). 

 This more dorsal portion of the mesoderm does not split up until 

 later, when there is formed in each segment another pair of 

 segmental cavities, the walls of which now meet in the dorsal middle 

 line (Laurie). "We thus find that the differentiation of the meso- 

 derm in the Scorpiones is very primitive, and strongly recalls the 

 similar process in the Annelida. A similar condition is also found 

 in the Araneae. Before the development of the primitive segments 

 has advanced thus far, the rudiment of the heart is said to appear. 



E. Blood-vascular System and Coelom. 



The heart, according to Ivowalevsky and Schulgin, develops from 

 the paired layer mentioned above as proceeding from the dorsal union 

 of the somatic and splanchnic mesoderm. These grow on either 

 side towards the dorsal middle line, where they unite. At the same 

 time they seem to bend upwards, each forming half of a tube open 

 towards the entoderm, which extends from the head to the tail of 

 the embryo. "When this half tube closes on its lower side, the 

 formation of the dorsal vessel is practically completed. The anterior 

 part of it, which lies in the cephalo-thorax, and the most posterior 

 part no doubt yield the anterior and posterior aortae. 



In the cavity of the heart there are many isolated cells which became 

 detached from the primitive segments before they extended dorsally. T 



