474 DEVELOPMENT OF THE CHICK. 



Examination of the blastoderm in front of the primitive 

 streak at about the sixteenth hour of incubation shows that the 

 epiblast is several cells thick (the medullary plate) and is 

 folded in the middle line to form the first trace of the nervous 

 system the * neural ' or ' medullary ' folds and that the 

 centre or axis of the head-process possesses for the hinder 

 part of its length a rod of cells the ' notochord ' already re- 

 ferred to. The mesoblast at present spreads out like a pair 

 of wings. The first definite organs to form are the nervous 

 system and the notochord. At the end of the first day, 

 therefore, the embryo is almost all head. 



3. The Second Day. 



On the first day a semicircular fold, or intucking of the 

 blastoderm (the ' head-fold ') had appeared in front of the 

 medullary plate, which makes great progress on this day, 

 and has the effect of folding off the anterior region of the 

 embryo from the underlying yolk. The formation of blood- 

 vessels on the surface of the yolk takes place at the same 

 time, and by the forty-eighth hour a circulation of blood 

 within and without the embryo is established. 



The chief changes in the embryo are increase in its length 

 and differentiation of the constituent layers. The develop- 

 ment of the brain is hastened on. The neural folds become 

 higher and meet in the mid-dorsal line, forming the neural 

 crest and tube. The anterior half of this tube expands to 

 form a succession of swellings the segments of the brain 

 whilst the spinal cord remains slender, and minute, closely seb 

 thickenings of the neural crest foreshadow the future ganglia. 

 Two of the brain swellings are especially noticeable. These 

 are optic vesicles and form the first trace of the eyes. Mean- 

 while the ears are indicated by a pair of depressions of the 

 epiblast in the region of the future hind-brain (fig. 86, EI). 



The most striking change made in the embryo during the 

 second day is the multiplication of the six or seven paired 

 blocks of tissue on either side of the nervous system and 

 notochord which had appeared on the first day. These 

 blocks or 'somites' are portions of the mesoblastic sheets 



