20 J. GRAHAM KERR. 



form as soon as they become separated from the hypoblast. 

 It consequently often happens that, when the sheet is con- 

 tinuous up to its edge, this edge with its rounded cells is very 

 sharply marked off from the hypoblast beyond. With only 

 such sections to go by one might well believe that the sheet 

 of mesoblast was quite independent of the hypoblast, and 

 growing inwards over its surface from the blastoporic rim 

 after the manner described for various forms by Lwoff, 

 Brauer, and others. It is at once seen from the study of a 

 complete series of stages, such as the above account is 

 based upon, that any appearance of the kind is quite 

 secondary, and that originally mesoblast and hypoblast rudi- 

 ments are perfectly continuous. I will return to this question 

 later on. 



With the formation of the medullary keel the mesoderm 

 sheet becomes thickened out to each side of it in the region 

 where the myotomes are to be formed. 



On account of the yolk-laden character of the mesoblastic 

 rudiment it is difficult to make out when its segmentation 

 begins. Distinct protovertebrse were first found in about 

 Stage 17, where there were six present. They were squarish 

 in section and were solid. 



Coelom. — The first parts of the coelom to appear are 

 myocoelic. In Stage 21 (PI. 4, fig. 21) a coelomic cavity is 

 seen to have appeared in the centre of the myotome. This 

 appears to arise by simple breaking down of the central cells, 

 the cavity not having at first any sharply-marked outline, 

 and irregular masses of yolk-laden protoplasm projecting into 

 it. A little later (Stage 23) the outline is quite definite 

 and the cavity is walled in by a single layer of regular 

 columnar cells. From this the coelom spreads outwards by 

 definite splitting. 



Early Development of Notochord. — The Notochordal 

 rudiment was left, (PI. 4, fig. 16) at a stage in which it remains 

 attached to the hypoblast on the separation of the mesoblast 

 from it on each side. It forms a median dorsal ridge running 

 along the middle line above the archenteric cavily. The 



