492 



EMBRYOLOGY OF THE CHICK 



In the myelencephalic portion of the brain, the neuromeres have lost 

 their dorsal constrictions, though they can still be seen on the lateral and 

 ventral surfaces, while the whole cord has thickened. This thickening- 

 constricts the lumen so that it is quite slit-like at this time. The neural 

 tube has closed completely at both anterior and posterior ends at this 

 period. 



It will be remembered that the neural or medullary plates have 

 formed, and their lateral folds have begun to unite to form the neural 

 groove. This union has now been completed. The ectoderm, dorsal to 



Diagrams of sections through the eye of the chick embryo at the end of the 

 second day. The dorsal margin is toward the top of the page in A and B. A. Eye 

 as viewed directly. B. Vertical section through the line x-cf, in A. C. Horizontal 

 section through the line y-y, in A. cf. Choroid fissure ; cv, cavity of primary 

 optic vesicle ; ec, superficial ectoderm of head ; i, inner or retinal layer of optic 

 cup ; I, lens ; o, outer or pigmented layer of optic cup ; ol, opening of lens sac from 

 surface of head ; pc, posterior chamber of eye ; s, optic stalk, continuous with the 

 floor and lateral wall of the diencephalon. (From Lillie "Development of the 

 Chick." by permission of Henry Holt & Co., Publishers.) 



the groove, has again become continuous, leaving a slight area between 

 neural groove and superficial ectoderm. 



It will also be remembered that there are small groups of cells on 

 each side of the midline lying within this area which we called neural 

 crests, to distinguish them from the neural folds with which they were 

 in close connection. 



The two crests lying on each side of the midline fuse for a time, but 

 because they began as two separate groups, they again become separate 

 in a short time. They also form a sort of column on each side of the 

 midline, running along the long axis of the embryo, but soon they seg- 

 ment and become the dorsal root ganglia or sensory ganglia of the spinal 

 nerves (Fig. 290). As the segmented portions of these neural crests also 

 extend into the head region, they there give rise to the ganglia of the 

 sensory cranial nerves. 



THE DIGESTIVE TRACT 



At the period we are describing, the fore-gut has extended from 

 the anterior intestinal portal as its posterior limit to the infundibulum 

 as its anterior limit. It is divided into a pharyngeal portion, lying ven- 



