AVKS. 



261 



becoming somewhat pear- 

 shaped, the broad (head) end 

 being anterior. In front of 

 the primitive streak a neural 

 (medullary) plate is formed 

 (cf. p. 224), upon which is a 

 longitudinal neural (medullary) 

 groove. This is bounded at 

 the sides by neural (medullary) 

 folds (Fig. 78, Mil), which 

 unite above to form the neural 

 tube. The folds first unite in 

 the region of the mid-brain. 

 The head now projects from 

 the surface of the yolk, and 

 makes with it, as seen from 

 the left side, an S -shaped 

 head-fold. A similar tall-fold 

 is formed later, and then 

 lateral folds. The embryo, at 

 first flattened out on the 

 curved yolk - surface, thus 

 gradually comes to assume 

 the form of the chick, and 

 the ever diminishing yolk re- 

 mains attached to its ventral 

 surface (Fig. 82). In a chick 

 at the end of the second day 

 of incubation (Fig. 79), the 

 cerebral vesicles (Fh, Mh, and 

 Hli) are present, and the eyes 

 and ears are commencing. 

 The front part of the brain 

 begins to bend round the end 

 of the notochord. At the end 

 of the third day, the front 

 part of the embryo is twisted 

 round so as to lie with its left 



Jiv 



streak ; vAf, head - fold, 

 either side of Rf. 



Fig. 78. SURFACE fc ViE\v OF CHICK, 

 RATHER LATER THAN FIG. 77 

 Magnified 39 diameters (from Kol- 



: liker). Mn, Medullary folds uniting 

 in the head region ; Pi', primitive 

 Three ''mesodermic somites are visible on 



