259 



CHAPTER XXIY. 



DEVELOPEMENT OF BIRDS. 



§ 171. The heat-force being converted into movements of the 

 parts of the germ thereto subjected, the expansion of tlie pellucid 

 area, fig. 133, a, is the first sign of such change : in tliis area ap- 

 pears the embryonal trace, in 

 the form of the parallel lines 

 called 'plica3 primitlva?,' which 

 diverge to form the cepha- 

 lic dilatations. Concurrently 

 with the appearance of the 

 myelencephalous columns, ib. 

 ^;,p, the blood-lakes expand in 

 the surrounding halones, and 

 \ tracts, ib. /i, A, along which 

 pass colourless blood-parti- 

 cles, extend from below the 

 cephalic expansion, Z*, to the 

 peripheral sinuses : as the pro- 

 to-vertebrai, ib. v, v, begin to 

 appear at the sides of the my- 

 elon, the red colour is ac- 

 quired by the blood, and the heart is made more manifest, by its 

 movements, as the * punctum saliens,' ib. c. A distinct membrane, 

 ' serous layer,' ib. s, s, is formed upon the germ and Ijlastoderm : ^ 

 the cephalic end of the embryo rises from the surface of the 

 blastoderm, and then cur^'ing doAvn, sinks into it, forming ibr 

 itself a kind of hood of the serous layer : it is reflected at b, to 

 show the fossa, /". This hood gradually extends from the margin 

 of the fossa over the body, and, meeting a similar fold formed 

 by the i)rojecting and incurved tail, closes over the germ on the 

 upper side, 'making a circumscribed cavity which is the amnios,''^ 

 fig. 134, a. The progress of differentiation of layers of the blasto- 

 derm has gone on beneath: in fig. 183, the ' serous layer' b is 



I Chu-k tliirty-fifth 



XX. vol. Y. p. XX. Ib. 1)1. Ixix. fig. 7. 

 s 2 



2 Ib. p. XX. 



