/--THAPTER VII 



THE STRUCTURE OF TWENTY-FOUR HOUR CHICKS 



The formation of the head; the formation of the neural 

 groove; the regional divisions of the mesoderm; the 



CCELOM, the pericardial REGION; THE AREA VASCULOSA. 



The Formation of the Head. — In embryos of 21 to 22 hours 

 the anterior part of the embryonal area is thickened and ele- 

 vated above the level of the surrounding blastoderm, with a 

 well defined crescentic fold marking its anterior boundary. 

 Between 21 and 24 hours this region has undergone rapid 

 growth (Fig. 15). Its elevation above the blastoderm is much 



,more marked and it has grown anteriorly so it overhangs the 

 proamnion region. The crescentic fold which formerly marked 

 its anterior boundary appears to have undercut the anterior 

 part of the embryo and separated it from the blastoderm. The 

 changes in relationships are due, however, not so much to a 

 posterior movement of the fold as to the anterior growth of the 

 embryo itself. This anterior region which projects, free from 

 the blastoderm, may now properly be termed the head of the 

 embryo. The space formed between the head and the blasto- 



• derm is called the subcephalic pocket (Fig. 17, E). 



In the mid-line the notochord can be seen through the over- 

 lying ectoderm. It is larger posteriorly near its point of origin 

 than it is anteriorly. Nevertheless it can be readily traced into 

 the cephaHc region where it will be seen to terminate somewhat 

 short of the anterior end of the head (Fig. 15). 



The Formation of the Neural Groove. — The neural plate in 

 chicks of 18 hours was seen as a fiat, thickened area of the ecto- 

 derm. In embryos of 21 to 22 hours a longitudinal folding had 

 involved it establishing the neural groove in the mid-dorsal 

 hne flanked on either side by the neural folds. At 24 hours of 



^ incubation the folding of the neural plate is much more clearly 

 marked. In a dorsal view of the entire embryo (Fig. 15) the 

 neural folds appear as a pair of dark bands. The folding which 



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