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 



/i.gr 



&o. 



■£t^t. 



-prjr. 



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 alreadv formed, Iving a short distance in front of the thickened 

 ectoderm destined to form the auditory pit. 



