THE VASCULAR AREA. 237 



beginnings of a vascular area, a pelucid area, and a germ wall, having a structure, 

 arrangement, and mode of growth similar to the corresponding ones in the 

 vertebrates. 



A comparison of the numerous sections given by S. Mollier to illustrate the 

 development of the blood will show that the structure and mode of growth of the 

 vascular cords in amphibians and cyclostomes are essentially the same as in 

 Limulus. 



The principal difference in our descriptions relates to the origin of the cells 

 in or near the germ wall. Many students of vertebrate embryology state that 

 yolk cells migrate into the germ wall and give rise to blood cells. In Limulus, 

 according to my description, while it is true that many cells originating in the germ 

 wall pass out of it into the yolk, there is no indication that the yolk cells migrate 

 in the opposite direction into the vascular cell cord. As the pictures presented 

 by the vascular cell cords in vertebrates and arachnids are identical, the difference 

 undoubtedly lies in the interpretations, not in the processes. 



It will be seen from an inspection of the diagrams illustrating these conditions 

 in mercator projection, that along the periphery of the germinal area in typical 

 arachnids such as the scorpion, the spiders, and Limulus, there is a zone of dividing 

 cells that constitutes one of the earliest and most important sources of blood cor- 

 puscles. (Fig. 138, av.) It may therefore be called the vascular area. Owing 

 to the forward growth of the abdominal lateral plates and the absence of such 

 plates in the thoracic region, a barren extra embryonic area is formed around the 

 head, which may be regarded as the beginning of a pellucid area. (Fig. 138, 

 B. a.pl.) 



It will be seen that when the same conditions are shown from the haemal sur- 

 face of the egg (Fig. 139, A,B.), the concrescing germ walls form a median band 

 or cord of yolk cells, heart cells, and blood cells, extending from the posterior 

 margin of the dorsal organ, or cephalic navel, to the caudal navel. 



In the vertebrates, we may recognize a modification of this condition, due 

 largely to the increase in the size of the yolk sphere. (Fig. 139, C.) When the 

 latter is of considerable size, the hsemal concrescence of the germ wall, in the 

 middle sections of the body, is delayed, or does not take place at all. Thus a 

 potential, or real, belly navel is formed, dividing the primitive vascular cord into 

 three sections. The definitive heart arises from the anterior section that lies 

 between the cephalic navel, or mouth, and the belly navel. In the middle section, 

 the vessel remains in a paired condition, forming around the navel a vascular 

 ring, the anterior part of which represents the Cuvierian ducts, or the proximal 

 ends of the vitelline veins. The posterior ends unite behind the belly navel to 

 form the posterior section of the primitive vascular cord, which is continuous, at 

 its posterior end, with the haemal lip of the blastopore, or caudal navel. From 

 the posterior section is formed the unpaired vitelline vein, which represents the 

 non-contractile caudal end of the arachnid haemal vessel. (Compare also Figs. 

 17. 23, 31. 34> 43-) 



