302 OUTLINES OF CHORDATE DEVELOPMENT 



condition at which we left them, for the most part about the 

 thirtieth hour. We shall limit this account to little more 

 than the bare mention of the more important morphological 

 alterations of structure, considering the systems separately. 



I. THE NERVOUS SYSTEM 



1. The Central Nervous System 



We have already traced the development of the central 

 nervous system up to the formation of the spinal cord and 



FIG. 122. Neuromeres in the brain of the chick embryo. Dorsal views. 

 From Von Kupffer (Hertwig's Handbuch, etc.) after Hill. A. Embryo of 

 twenty-four hours; two pairs of somites. B. Embryo of twenty-four hours; 

 five pairs of somites. C. Embryo of twenty-five and one-half hours; six pairs 

 of somites. D. Embryo of twenty-six hours; seven pairs of somites, cp, 

 Posterior limit of prosencephalon; fr, posterior limit of mesencephalon; ps, 

 primitive streak; u, first somite; 1-11, neuromeres. 



brain, and the establishment, in the latter, of the primary 

 hind-, fore-, and mid-brains. In connection with the pri- 

 mary fore-brain we noted the rudiments of the optic vesicles, 

 and in the primary hind-brain the series of six neuromeres, 

 or brain segments, which are clearly distinguishable at the 

 thirty-hour stage .described. 



