DEVELOPMENT OF THE CHICK FIEST PEEIOD. 299 



Fig. 327. Yolk of the natural size 

 after eighteen hours of incubation : a. 

 vitellus; b, area pellucida; c, area 

 vasculosa. 



area presents a pyriform outline (figs. 327 and 328) 

 the canal for the spinal 

 cord, which is bounded by 

 the dorsal laminae, we ob- 

 i- the chorda dorsalis 

 the dorsal cord (figs. 330 

 and 332, A, e, and fig. 

 331, /), an extremely fine 

 elongated streak, surround- 

 ed by a transparent sheath; 

 both* the dorsal cord and 

 the sheath go to constitute 

 the cartilaginous column 

 which appears later, and out 

 of which, by its becoming 

 divided into pieces, the ver- 

 tebral column is produced 

 ( 466). The embryo with 

 its laminae dorsales now 

 bends itself forward, at the 

 same time that it here forms 

 a sickle-shaped transparent 

 fold (fig. 328, c), the future 

 intolvcrum capitis the 

 cranial envelope or cap. 

 From the twentieth to the 

 twenty-fourth hour, the 

 transparent germinal area 

 is observed to become 

 longer and more fiddle- 

 shaped. The cristae, or 

 folds of the dorsal laminae, 

 where they run closest to- 

 gether, appear somewhat 

 sinuously bent (fig. 331, 

 b,b} ; here, too, in the pecto- 

 ral region, on both sides of 

 the dorsal laminae, near 

 their cristse, there appear 



Unde] 



dark, four-cornered looking veloped. 



Fig. 328. The 'pellucid area of 

 fig. 327 magnified ; a, the pellucid 

 area, now become pear-shaped ; in- 

 stead of the nota, or primary streak, 

 the two dorsal laminae or folds (lami- 

 na 8. plicae, dorsales) b, b, are seen ; 

 the involucrum capitis, or cranial en- 

 velope, c, a falciform fold, or kind of 

 reflex blastoderma, begins to be de- 



