524 EMBRYOLOGY OF Till- LoWKi; YKh'TKUKATKS CH, 



derm lyini: outside tlie boundary of the pellucid area there are 

 present the same layers of cells as in the pellucid area the ectoderm, 

 which extends farthest peripherally, the endoderm which passes into 

 a thick yolk svn< -vtial layer peripherally (germinal wall), and the 



n.r 



my. 



me. 



ect. 



spLc. 





***** 



I sow 



end 



ac 



Fi<.. 228. Transverse section through the body of a Fowl embryo about the end of the 

 Hist day of incubation. 



ect, ectoderm ; end, endoderm ; me, myocoele ; my, myotome (mesoderm segment) ; JV, notochord ; 

 n.r, neural rudiment ; sow, somatopleure ; spl, splanchnopleure ; splc, splanchnocoele. 



mesoderm the outer part of which is still unpenetrated by the 

 coelomic split. The part of the opaque area where mesoderm is 

 present assumes a very characteristic mottled appearance (Fig. 227, 

 C, a.v) caused by the rudiments of blood-vessels and blood : hence 

 the name vascular area which is given to this part of the blasto- 

 derm. When the embryo has reached the stage with about seven 



mesoderm segments the secretion 

 of tin id (plasma) commences within 

 the blood islands. 



THE SECOND DAY OF INCUBATION. 

 The general appearance of an egg 

 opened during the second day of 

 incubation is seen in Fig. 229. Tin 1 

 blastoderm has increased consider- 

 ably in si/o and now covers about 

 110. The pellucid area has assumed 

 a somewhat liddlc-like shape. 



On examining the excised blasto- 

 derm about (he ColimieiieeimMit of 

 lbi> day it is seen thai the formation 

 of tin- head fold lias progressed COD 

 urably and the head rudiment 

 prnjct-i< more OpnSplcUOUSlj ahn\v 

 in-ral h-vrl o| i| M - blastoderm, Within tin- head rudiment 

 . refill can he seen and it is noticeable that it stretches 

 farther hark than doeg tin- mil.-r \\all !' the head rudiment. In 

 ntlu-r \sortl- the head !'..|d .!' the eiidndenn hassjirrad lartherbaek 





; Fl<j. ! :il.,iit tlit- 



middle ..I tin- ^^.()||(1 d:iy <!' iiiciil.;iti<.n. 



Mk In i In- ci-ntu- i> 



-a, with tin- Mi.lim.Mit (,r 



. 



