108 THE DEVELOPMENT OF THE CHICK 



brain. The first clear landmark is given by the mesoblastic so- 

 mites, because it is known that the four anterior somites are 

 cephalic. All of the neural tube in front of the fifth somite is 

 therefore cranial. What a large proportion of the neural tube 

 this is in early stages may be seen by comparison of figures of 

 embryos in the period covered by the chapter (cf. Fig. 61). Be- 

 fore the appearance of the first somite the entire medullary plate 

 in front of the primitive streak is in fact cranial. 



Origin of the Primary Divisions of the Embryonic Brain. The 

 embryonic brain is divided into three divisions of unequal length, 

 viz., the fore-brain (prosencephalon), mid-brain {mesencephalon), 

 and hind-brain (rhombencephalon). The first division is character- 

 ized in the period we are considering by its very considerable 

 lateral expansions, the rudiments of the optic vesicles (Figs. 59, 

 61, 63, etc.), and also by the fact that there is a suture in the 

 anterior portion of its floor owing to the mode of its origin (Fig. 

 62). A definite constriction between it and the following division 

 first appears in embryos Avith six or seven somites (Fig. 59). At 

 the stage of 9-10 somites the next division (mid-brain) becomes 

 clearly marked off by a constriction from the hind-brain (Fig. 

 61). The latter is relatively very long, and its anterior half is 

 characterized in the 12-somite stage by the existence of five divi- 

 sions (neuromeres) separated by constrictions (Fig. 63). 



It will be noted that the first neuromere of the hind-brain appears 

 about twice as large as the succeeding ones ; it really includes two neuro- 

 meres according to some authors. Similarly, it is maintained that the 

 mid -brain includes two neuromeres and the fore-brain three. 



According to Hill's account the entire brain of the embrj^o chick 

 is composed of eleven neuromeres or neural segments, which are formed 

 even in the 1 s stage. The first three enter into the composition of the 

 fore-brain; the next two, viz., 4 and 5, form the mid-brain, and the last 

 six the hind-brain. 



The three that enter into the composition of the primary fore-brain 

 have the following fate according to Hill: the first forms the telen- 

 cephalon, the second the anterior division (parencephalon) and the third 

 the posterior division (synencephalon) of the diencephalon. The cere- 

 bellum arises from the first neuromere of the hind-brain, sixth of the 

 series. This question is more fully discussed in Chapter VI. (See 

 Fig. 83.) 



