76 ARTHUR DENDY. 



on either side of the alimentary canal (fig. 66). The caloni 

 within the embryo develops in the ordinary manner as a split 

 between the somatopleuric and splanchnopleuric layers of 

 mesoblast (cf. figs. 66 — 69) ; this is directly continuous with 

 the large pleuro-peritoneal space outside the embryo, which 

 develops chiefly as a split between the serous envelope and 

 the yolk-sac (cf. figs. 68—72). 



The head cavities appear at Stage J as two small spaces in 

 the mesoblast just outside the aortic dilatations on either side 

 of the anterior extremity of the alimentary canal (fig. 62). 

 At Stage K they are visible from the outside as dark patches 

 behind the paired eyes (figs. 74<, 75), and each has begun to 

 divide into anterior and posterior portions (fig. 77). At Stage 

 L they have become very large (fig. 91), and those of the an- 

 terior pair are connected across the middle line by a short trans- 

 verse canal (fig. 90, H. C. C), while each of the posterior pair 

 gives off a conspicuous branch into the mandibular arch (fig. 91, 

 H. C, 31.). The head cavities are lined by short columnar cells. 



The Mesoblastic Somites. — The vertebral and lateral 

 plates of mesoblast appear to be derived, at any rate mainly, 

 from the great sheet of mesoblast which grows forwards 

 from the primitive streak (compare Stage D, figs. 11 and 

 12; Stage E, figs. 29—34; Stage J, figs. 68—71). Trans- 

 verse sections of embryos of Stage E already, perhaps, show 

 indications of the division of the mesoblast into vertebral and 

 lateral plates (figs. 29 — 31 and 33), but it is not until Stage H 

 that the vertebral plate begins to be segmented into meso- 

 blastic somites or protovertebrse. The mesoblastic somites do 

 not appear with the same regularity as in the chick, so that 

 they are of little use for the purpose of classifying the embryos 

 in stages. Generally speaking, they may be said to develop 

 from before backwards, though a number of them seem to 

 appear almost or quite simultaneously at first. The coelomic 

 split between somatopleure and splanchnopleure at first ex- 

 tends into them (fig. 69), but they soon separate completely 

 from the lateral plates, forming squarish blocks composed of 

 radially elongated columnar cells (fig. 68). 



