DEVELOPMENT OF THE CHICK FIRST PEKIOD. 301 



cond day of incubation the embryo disconnects itself c-ven 

 more and more from the ger- 

 minal membrane and the 

 yolk, and rises more dis- 

 tinctly over the germinal 

 area. This takes place by 

 the anterior plait or fold (in- 

 volucrum capitis) continu- 

 ing to recede still farther 

 backwards (fig. 334, e}, and 

 the development posteriorly 

 of a second plait or fold, 

 sickle-shaped or crescentic 

 in the first instance also (fig. 

 334, g), the future involu- 

 crum caudce; the sides now 

 begin to turn inwards also, 

 by which the transparent 

 germinal area is drawn in 

 and bent laterally, and 

 made to assume a complete 

 fiddle-shape (figs. 333 and 



334). The embryo is three 



' . , ,{ , Fig. 331, Magnified view of the 



lines m length ; the broader pellucid area of the yolk> fig> 330 . the 



and more strongly bent ex- area has now lost its pear-shape in a 

 treniity, with its transverse great degree, and become somewhat 

 plait or envelope, is visible fiddle-shaped (biscuit-shaped in the 



original). In the middle are seen the 

 slightly sinuous edges of the dorsal 

 lamina, b, b, separating from one ano- 

 ther anteriorly and posteriorly ; on 

 their outsides lie four square plates, 

 c, c, rudiments of the vertebral co- 



J umn ' * * nt f or , c ^ ebral c ! u . '. e > *> 

 . , , transparent edge of the cranial invo- 



completely, close the canal lucrum> shining through; /, dorsal 

 for the spinal cord (fig. 335, cord. 

 A, g], beneath which the 



more delicate chorda dorsalis with its sheath (e) extends. The 

 four-cornered laminae, the future vertebral arches, have in- 

 creased in number, new ones springing up in front and be- 

 hind ; and, about the thirty-sixth hour, as many as from ten 



to the naked eye. The cris- 

 tae of the dorsal laminae 

 have become approximated 

 through a larger space, 

 touch each other (fig. 334, 

 b, 6), and finally coalescing 



