462 \V. B. SCOTT AND HENRY F. OSBORN. 



changes in the yolk cells proper. We shall refer to this as 

 the yolk hypoblast. 



The growth of the former class has been already con- 

 sidered in full. The latter arises by a slow process of meta- 

 morphosis in the peripheral yolk cells. The changes are 

 not difficult to follow. The square yolk cells split as they 

 approach the surface iiilo long columnar or oblong cells, and 

 at the same time a change takes place in the yolk spherules 

 with which they are loaded, so that they show a greater 

 avidity for the staining fluid. The large spherical nuclei of 

 the yolk cells give place to the characteristic oval nuclei of 

 the hypoblast. These primitive hypoblast cells assume more 

 regular proportions as development proceeds. In the split- 

 ting off of the mesoblast which soon follows, fresh cells are 

 constantly supplied from the yolk. 



A further notice of Calberla's^ views upon these points will 

 perhaps not be out of place here. He considered the Lam- 

 prey embryo immediately after invagination to consist of two 

 layers, the primary entoderm and the ectoderm. The former 

 divides everywhere, except across the axial line, into the 

 secondary entoderm and the mesoderm. Across the axial line 

 the primary entoderm remains intact. He does not admit that 

 the mesoderm arises even in part by invagination ; but, still 

 more important as it bears on the question under discussion, 

 he does not include the outer yolk cells as part of the pri- 

 mary entoderm. So what we shall consider hereafter as the 

 lateral mesoblast, he concluded, was joint mesoblast and 

 hypoblast, not allowing that the outer yolk cells formed a 

 distinct layer. The comparison has been inserted because at 

 this period of its history the Lamprey presents many points 

 in common with the Newt. 



. To resume the study of the hypoblast in Triton, it may 

 be considered in the latter part of Stage c as forming a con- 

 tinuous layer around the yolk and completely enclosing the 

 alimentary canal. By Stage b a very decided change has 

 taken place (see Fl. XX, fig. 5), The section is in the head 

 region where the alimentary tract has now reached a con- 

 siderable size. The hypoblast is now only in contact with 

 the epiblast in the median line, although the connection is 

 such a close one that the three or four cells, still adhering, im- 

 pinge so closely as to form a decided indentation in the 

 epiblast — a feature which has been previously noticed in the 

 Elasmobranch Fishes. The middle cells have also elongated 

 and narrowed considerably, while those at the sides remain 

 shorter ; this results in a rounded upper outline. Laterally, 

 ^ Vide I!,. Calberla, loc. cit., ou ' Tdromyzon jilaneri^ 



