460 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



the region adjacent to the neural tube^ spht into two layers separated 

 by a space containing fluid. This space is the coelome (Fig. 193, splc). 

 The outer or somatic layer of the mesoderm is applied closely to the 

 ectoderm to form with it the body wall or somatopleure (Fig. 193, som) 

 while the inner or splanchnic layer is applied to the endoderm, forming 

 with it the primitive gut wall or splanchnopleure {spT). The mesoderm 

 adjacent to the neural tube is much bulkier than it is elsewhere, and this 

 portion undergoes a process of segmentation into successive blocks lying 

 one behind the other — the embryonic rnyotomes (Fig. 193, my). These 

 increase in number, new myotomes being added on at the hinder end of 

 the series. 



The portion of the embryo containing the brain, i.e. what will become 

 the head region, bulges well above the general surface and is marked 



Fig. 193. 



Transverse section through Fowl embryo of the second day (15 myotomes). A, Paired dorsal 

 aorta ; a.n.d^ archinephric duct ; ect, ectoderm ; endj endoderm ; my, myotome ; N, notochord ; 

 s.c, spinal cord ; som, somatopleure ; spl, splanchnopleure ; splc, splanchnocoele. 



off by the blastoderm being tucked in underneath it in front and at the 

 sides. Into this head rudiment there projects forwards a wide tube of 

 endoderm (Fig. 192,/.^) which has been tucked off from the general 

 endoderm in precisely the same way as the ectoderm forming the outer 

 wall of the head rudiment has been tucked off from the general ectoderm. 

 This tube of endoderm is the rudiment of the anterior portion of the 

 alimentary canal or fore-gut. Immediately underneath the fore-gut is a 

 narrower tube bifurcating at its hind end like a A. This is the rudiment 

 of the heart, and the two branches into which it divides at its hinder 

 end are the two vitelline veins which return the blood from the surface 

 of the yolk. By this time the heart is usually commencing to grow 

 actively in length with the result, owing to its being fixed at each end, 

 that it bulges out on the embryo's right side. About the commencement 

 of the second day of incubation the inner portion of the opaque area 

 (Fig. 192, a.v) begins to assume a characteristic mottled appearance due 



