THE NERVOUS SYSTEM. 



463 



divisions but have a certain value for descriptive purposes. The cavities of 

 the brain are the ventricles and their connecting passages, namely, the third 

 ventricle of the diencephalon and the fourth ventricle of the rhombencephalon, 

 the two being connected by the mid-brain cavity (aquceductus Sylvii}. The 

 telencephalon usually develops a more or less paired character, its cavities 

 being then paired diverticula of the unpaired fore-brain cavity and known as 

 the lateral ventricles. 



Before the closure of the brain part of the neural tube, transverse constric- 

 tions appear across the neural plate. The transverse rings into which the 



-ps. 



FlG. 403. Chick embryos; i, of 22 hours' incubation; 2, of 24 hours; 3, of 25^ hours; 4, of 26 



hours, showing respectively 2, 5, 6, and 7 primitive segments. Hill. 



cp., Caudal limit of fore-brain; //., caudal limit of mid-brain; ., first primitive segment; 



ps., primitive streak; i-n, neuromeres. 



tube, when completed, is thus divided are known as neuromeres. They are 

 held to represent a primitive segmentation of the head, similar, perhaps, to 

 that exhibited by the spinal nerves and segmental somatic musculature (primi- 

 tive segments) of the trunk. The neuromeres may appear before the head 

 somites. To what extent they correspond to the somites or to the visceral 

 segmentation (p. 467) and also to the cranial nerves is a matter of dispute. 

 Concerning their number there have been various views, the evidence inclining 

 to three in the fore-brain, two in the mid-brain and six in the rhombic brain 

 (Fig. 403). Their presence and number are most in doubt in the cephalic end 

 of the tube, the highly modified prosencephalon. 

 30 



