132 Elementary Zoology. 



At the same time a split appears on each side of the body, 

 parallel to the notochord, which sepa,rates the mesoblast into a 

 vertebral and a lateral portion, while transverse clefts break up 

 the vertebral portion of the mesoblast sheet into a series of 

 squarish blocks, the protovertebrsB, or, better, " mesoblastio 

 somites." At this stage, therefore, the embryo of the fowl is 

 distinctly segmented, and, it will be observed, the portion of 



/NE 



Fig. 69.— Transverse section of Chick at 48ih hour. (From Marshall.) 



NE, spinal ganglion ; ns, spinal chord ; CH, notochord ; MP, muscle space with contained 

 ccelom (cm) ; yc, posterior cardinal vein ; kc, Wolffian duct ; ks, nephrostome ; OE, 

 genital epithelium; c, ccelom; A, aorta; an, amnion; me, somatopleure ; mh, 

 splanchnopleure ; h, hypoblast ; vv, vitelline vein. 



the ccelom lying within the segmented portions of mesoblast is 

 also segmented. There are, in fact, a series of ccslomic spaces 

 as in the earthworm. 



The fowl agrees with reptiles and all animals higher in the 

 scale, to differ from the frog and other animals lying lower in 

 the scale of vertebrated animals in the possession of an organ 

 of protection for the embryo, which is known as the amnion. 

 The first trace of this to appear is the head-fold (Fig. 69, an), 

 an upgrowth of epiblast first, but later lined with the somato 

 pleuric mesoblast ; then there is a corresponding tail-fold and 

 two lateral folds— these meet and form a thin but double-layered 



