94 



THE DEVELOPMENT OF THE CHICK 



of very flat cells. The lateral extensions may be regarded as 

 diverticula; subsequently these grow more rapidly at four places 

 along their length, and come in contact with the ectoderm. Thus 

 four pouches are established on each side as described in detail 



HE - 



FIG. 48. Stage of first intersomitic groove 

 drawn from an entire mount in balsam by 

 transmitted light. 



a. c. v., Amnio-cardiac vesicle, a. o., In- 

 ner margin of Area opaca. Ect., Ectoderm. 

 Ent., Entoderm H. F., Head -fold, i.s.f.l., 

 First intersomitic furrow, med . pi . , Medullary 

 plate. Mes., Mesoderm. n. gr., Neural groove, 

 pr. gr., Primitive groove. Pr'a, proamnion. 



in the next chapter. At the 12 s stage one such place of contact 

 is already formed, lying a short distance in front of the thickened 

 ectoderm destined to form the auditory pit. 



