FOUR TO Six SOMITE STAGE 467 



fusion of the lower or inner margins of 'these surfaces, the upper >nes 

 again forming a continuous ectoderm, so that the neural tube becomes 

 entirely separated from it. The cells lying between this ectodermal and 

 lower margin, and which have been derived approximately from the 

 apices of the neural ridges, become the neural crests (Fig. 272). These 

 crests do not fuse in the midline, but remain as a pair- of longitudinal 

 bands along the dorsal-lateral surfaces of the neural tube, and are the 

 rudiments of the ganglia of the cranial and spinal nerves. They are not 

 uniformly developed, and appear much clearer in some sections than in 

 others. 



Already very early in the area opaca considerable modification has 



been going on. The area itself has 

 broadened and in both lateral and 

 posterior regions^it is mottled. This 

 mottled portion Ts due to jthe-Jprm- 

 ing of differentiated cell groups 

 forming what are called blood- 

 islands (Figs. 262, 264), which are 

 .the beginning of the vascular sys- 

 tem. The portion of the area opaca 

 in which these blood-islands occur 

 is known as the area vasculosa. This 

 area vasculosa begins immediately 



SapmbrBthl ptc 



Fig 2?2 - behind the embryo, but then extends 



Transverse section through the head of a laterally and anteriorly, while 



7 day Ammoccetes in the region of the tri- . . i .-: . : t , r . 



geminai ganglion, von Kupffer. around its periphery a single definite 



blood vessel forms, known as the sinus terminalis (Fig. 284, C). Beyond 

 this sinus terminalis, all the remaining area opaca consists of ectoderm 

 and entoderm, and extends around the yolk. It is then called the area 

 vitellina. 



-The compact cell masses forming these blood-islands have been 

 formed throughout the deeper portion of the germ wall, becoming cov- 

 ered superficially with a deep layer of scattered germ wall cells. This 

 superficial layer comes to be known as coelomic "mesoderm." 



This superficial layer and the blood islands become continuous very 

 early with the mesoderm of the pellucid area derived from the primitive 

 streak (Fig. 273). The blood islands become hollowed out, forming 

 lacunae. The cells which have formed the blood islands become both 

 the blood-vessel walls arid the blood cells. The lacunae then anastomose, 

 forming a complete network extending to meet the vascular structure of 

 the pellucid area .and later of the embryo. 



!o -The cellular portion of the germ wall which remains after the 

 coelomic "mesoderm" and the blood islands have differentiated, forms 

 the rudiment of the yolk-sac entoderm, to be described later. 



As soon as 'the extra-embryonic coelom forms, dividing the meso- 



