STRUCTURE OF FOUR-DAY CHICKS 113 



are formed by bud-like outgrowths from somites. The anterior 

 appendages arise opposite somites 17 to 19 inclusive, and the 

 posterior appendages arise opposite somites 26 to 32 inclusive. 

 During the fourth day the appendage buds increase rapidly in 

 size and become elongated but otherwise their appearance and 

 their relationships show little change. 



The Allantois. The development of the extra-embryonic 

 membranes has already been considered (Chap. XI) and needs 

 no further discussion here. In order to show the embryos more 

 clearly, the extra-embryonic membranes, except for the allan- 

 tois, have been removed from the specimens drawn in Figures 

 39 and 40. The cut edge of the amnion shows at its anterior 

 attachment to the body, opposite the anterior appendage bud 

 and just caudal to the tip of the ventricle. The allantois in 

 the three-day chick is as yet small and is concealed by the pos- 

 terior appendage buds. In four-day embryos it has undergone 

 rapid enlargement and projects from the umbilical region as a 

 stalked vesicle of considerable size. 



II. THE NERVOUS SYSTEM 



Summary of Development Prior to the Third Day. The 



earliest indication of the formation of the central nervous sys- 

 tem appears in chicks of 16 to 18 hours as a local thickening of 

 the ectoderm which forms the neural plate (Fig. n). The 

 neural plate then becomes longitudinally folded to form the 

 neural groove (Figs. 14 and 15). By fusion of the margins of 

 the neural folds, first in the cephalic region and later caudally, 

 the neural groove is closed to form a tube and at the same time 

 separated from the body ectoderm. The cephalic portion of 

 the neural tube becomes dilated to form the brain and the re- 

 mainder of the neural tube gives rise to the spinal cord (Figs. 

 18 and 21). 



In its early stages the brain shows a series of enlargements 

 in its ventral and lateral walls, indicative of its fundamental 

 metameric structure. In the establishment of the three vesicle 

 condition of the brain, the lines of demarcation between pros- 

 encephalon, mesencephalon, and rhombencephalon are formed 

 by the exaggeration of certain of the inter-neuromeric constric- 

 tions and the obliteration of others (see Chap. IX and Fig. 20). 



