524 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



derm lying outside the 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 syncytial layer peripherally (germinal wall), and the 



my. 





eel. 



% 



splc 



~*fegsSS5fc- •■ 





ftHW/'^W^V **'-*.*» 4-;- 





\som. 

 >pl 



**■«&!*» 



»**«*• 



N. 



end. 



Fig. 228. — Transverse section through the body of a Fowl embryo about the end of the 

 first day of incubation. 



ect, ectoderm ; end, endoderm ; mc, myocoele ; my, myotome (mesoderm segment) ; N, notochord ; 

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



mesoderm the outer part of which is still impenetrated 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 fluid (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. The 

 blastoderm has increased consider- 

 ably in size and now covers about 

 110°. The pellucid area has assumed 

 a somewhat fiddle-like shape. 



On examining the excised blasto- 

 derm about the commencement of 

 this day it is seen that the formation 

 of the head fold has progressed con- 

 siderably and the head rudiment 

 projects more conspicuously above 

 the general level of the blastoderm. Within the head rudiment 

 the foregut can be seen and it is noticeable that it stretches 

 farther back than does the outer wall of the head rudiment. In 

 other words the head fold of the endoderm has spread farther back 



Fio. 229.— Egg of the Fowl about the 

 middle of the second day of incubation. 



a.o, circular opaque area. In the centra is 

 the dark pellucid area, with the rudiment of 

 the embryonic body lying along its axis. 



