66 THE FIRST DAY. [CHAP. 



amongst them. The nuclei, some of which are probably enclosed 

 by a definite cell body, typically form a special layer immedi- 

 ately below the epiblast. A special mass of nuclei (vid-e Tigs. 18 

 and 20, n) is usually present at the junction of the hypoblast 

 with the germinal wall. 



The germinal wall retains the characters just enumerated till 

 near the close of the first day of incubation. One function of its 

 cells appears to be the absorption of yolk material for the growth 

 of the embryo. 



The chief events then of the second period of the 

 first day are the appearance of the medullary folds 

 and groove, the formation of the notochord and lateral 

 plates of mesoblast, the beginning of the head-fold and 

 amnion, and the histological changes taking place in the 

 several layers. 



Prom the 20th to the 24th hour. A view of 

 the embryo during this period is given in Fig. 23. 

 The head-fold enlarges rapidly, the crescentic groove 

 becoming deeper, while at the same time the over- 

 hanging margin of the groove (the upper limb of the 

 g), rises up above the level of the blastoderm ; in fact, 

 the formation of the head of the embryo may now be 

 said to have definitely begun. 



The medullary folds, increasing in size in every 

 dimension, but especially in height, lean over from 

 either side towards the middle line, and thus tend 

 more and more to roof in the medullary canal, espe- 

 cially near the head. About the end of the first day 

 they come into direct contact in the region which 

 will afterwards become the brain, though they do not 

 as yet coalesce. In this way a tubular canal is formed. 

 This is the medullary or neural canal (Fig. 23, Fig. 24, 



