EARLY DEVELOPMENT OF JULUS TBREBSTRIS. 227 



The keel is shown in transverse section in fig. 9 a, and 

 fig. 20. At the end of the sixth day the keel is still present 

 but an alteration is taking place in the cells of which it is com- 

 posed. They are no longer round and thick, but are becoming 

 elongated in the direction parallel to the surface. At the same 

 time they continue to multiply and spread themselves out, so 

 as to form two definite layers within the ectoderm (fig. 10). 

 These are the splanchnic and somatic layers of the mesoderm. 

 The cells of the ectoderm and of the somatic mesoderm are 

 still connected, and also the cells of the splanchnic and somatic 

 mesoderm. 



On the eighth day the mesoderm extends round a great part 

 of the embryo — rather more than half way round. The keel 

 has almost disappeared (fig. 11). 



On the ventral surface the cells are no longer flat but have 

 assumed a columnar form. Their nuclei are now oval in shape, 

 their long axis pointing, as does that of the cells to which they 

 belong, towards the interior of the ovum. This is in fact the 

 first formation of the ventral plate and is shown in fig. 10. 

 While these changes are going on the remnants of the keel are 

 disappearing. The mesoderm now becomes thicker on each 

 side of the ventral line. This is shown in fig. 21. Both 

 layers are concerned in this thickening, and at these points the 

 two layers become indistinguishable. Outside the thickenings, 

 that is, farther away from the middle ventral line, the two 

 layers are closely applied to each other and to the epiblast as 

 before. The efFect of these changes is that the greater part of 

 the mesoderm is now arranged in two parallel longitudinal 

 bands along the ventral surface of the embryo ; these bands 

 being connected in the middle line by a thin portion consisting 

 of two layers (fig. 22). Fig. 21 is a transverse section through 

 the ventral half of an ovum at this stage. 



The two longitudinal bands now begin, to be constricted ofi' 

 into the mesodermal somites. The latter are formed from 

 before backwards and their position corresponds with that of 

 the future segments of the body. The number of somites thus 

 formed is eight, corresponding to the eight segments with 



