THE EARLY DEVELOPMENT OF THE CHICK 271 



siderably. Its most anterior region expands transversely, the 

 lateral extensions thus formed representing the rudiments of 

 the optic vesicles (Figs. 106, 107). This is obviously, therefore, 

 the region of the primary fore-brain or prosencephalon, which 

 now includes the entire anterior portion of the nerve tube. Its 

 posterior limit is marked by a slight constriction just back of 

 the optic vesicle rudiments. A little later (9-10 pairs of so- 

 mites) another constriction appears marking the posterior limit 

 of the mid-brain or mesencephalon. The third section of the 

 brain, the hind-brain or rhombencephalon is much the longest 

 section of the brain (Fig. 107). It is marked by a series of 

 irregular constrictions, ultimately five in number forming six 

 segments, or neuromeres, in this region. No posterior limit of 

 the hind-brain can be made out in these early stages, but from 

 later development it is known that all in front of the fourth 

 mesodermal somites really belongs to the head, and conse- 

 quently this level may be taken to mark approximately the 

 posterior limit of the brain. The hind-brain narrows poste- 

 riorly and the remainder of the neural tube is the rudiment of 

 the spinal cord, which remains narrowed and approximately 

 uniform in diameter. 



5. Vascular System 



We may now return to a description of the formation and 

 development of the embryonic vascular system, postponed 

 from an earlier page. The formation of the vascular rudiments 

 in the area opaca continues, in the manner already described 

 (Fig. 95), and when the extra-embryonic coelom forms, dividing 

 the mesoderm into somatic and splanchnic layers, the blood- 

 vessels remain associated with the latter, i.e., with the splanch- 

 nopleure (Fig. 105, A). The vessels of the pellucid area 

 develop first from its margin, near the area opaca, and gradually 

 extend toward the embryo. They first appear shortly after 

 the head-fold, just as the somites begin to be cut out. The 

 formation of the conspicuous blood islands does not occur in 

 the pellucid portion of the blastoderm; only the tubular vessels 



