262 COMPARATIVE ANATOMY 



and an inner or splanchnic layer adjacent to the hypoblast. 

 The narrow cavity between these two layers is the body-cavity 

 or ccelom. At the time of its first appearance it extends some 

 way into the segmental plates, but the latter soon become solid, 

 and the ccelom is confined to the lateral plates. 



As soon as the neural folds begin to turn inwards to form 

 the nerve-tube, the segmental plates begin to be divided by 

 transverse lines into a series of paired blocks or mesoblastic 

 somites, and shortly after their appearance the somites are 

 divided off from the lateral plates of mesoblast, into which 

 the segmentation does not extend. The first somite is formed 

 at some little distance from the anterior end of the body, 

 and new somites are formed successively from before back- 

 wards. The head region lies in front of the first somite, and 

 here the mesoblast is not segmented, but breaks up into a 

 mass of scattered branched cells, united to one another by 

 protoplasmic processes. Such scattered branched mesoblastic 

 cells are commonly called mesenchyme, and it should be noted 

 that the head of the frog is never segmented, even in the 

 embryonic condition, whereas, as we have learned (p. 218), the 

 head of the embryo dogfish is definitely segmented, the seg- 

 mentation disappearing in later life. It is worthy of remark 

 that there are traces of a pair of mesoblastic somites in front 

 of the first pair of body somites in the embryo-frog, but these 

 soon break up to form mesenchyme. We have here an 

 instance of abbreviation in development. The frog's head, 

 instead of passing through a segmented condition which is 

 subsequently lost, passes at once to the unsegmented con- 

 dition of the adult, though there are just traces enough to 

 remind us of a primitive segmentation. 



The embryo has now reached the stage snown in sagittal 

 section in fig. 63, which has been so fully lettered that a 

 lengthy description is unnecessary, and a dorsal view of the 

 same stage is shown in fig. 60. Only the ventral part of the 

 mesoblast appears in the sagittal section, as the somites and 

 lateral plates of the mesoblast lie to the sides of the median 

 plane cut by the section. It should be noted that there is 

 an anus, but no mouth, and that the notochord only extends 

 as far forward as the floor of the mid-brain, the fore-brain 

 being bent down over its anterior end to form the neural 

 flexure, which is afterwards straightened out. The embryo 



