1 88 Ed-ivin G. Conklin. 



derm cells remain on the rounded surface of the embryo. I have 

 not observed in detail the process of gastrulation in any of these 

 quarter embryos, but it is evident that there is no considerable 

 gastrula cavity and that the endoderm cells are chiefly overgrown 

 bythe ectoderm, as show^n in Fig. 65. Ultimately the endoderm and 

 mesoderm are largely overgrown, though in this case, as in the half 

 embryos, the ectoderm does not entirely inclose the embryo on the 

 side next to the injured cells and through the opening thus left 

 some of the endoderm cells may protrude. 



Although the localization of ooplasmic substances and of organ 

 bases is usually the same as in the quarter of an entire embryo, 

 in some cases there are dislocations of these substances and 

 bases which are probably due to injury of the surviving quar- 

 ter. Thus in the left anterior quarter, shown in Fig. 64, large 

 endoderm cells lie at the surface next to the first cleavage 

 plane; in the same quadrant of another egg shown in Fig. 65 the 

 neural plate cells lie at the periphery of the quadrant and chiefly 

 on the left side, instead of along the median plane as in normal 

 embryos. I have seen many other instances of such dislocations 

 but they are all of such nature that they can be interpreted as due 

 to slight injury to the surviving blastomeres. In not a single 

 instance are parts derived from a blastomere which would nor- 

 mally have come from another cell. 



The anterior quarter embryos are always recognizable by the 

 presence of the neural plate and, in later stages, of the sense spots. 

 The neural plate usually remains at the surface and is not infolded, 

 but in some cases it is invaginated through at least a portion of its 

 area, though a sense vesicle is not formed. (Figs. 63, 66.) In all 

 later stages one or more sense spots appear in the plate. (Figs. 

 66-70.) The neural plate always lies along the dorsal side of the 

 embryo, though it may be shifted more or less from the median 

 plane. (Figs. 65-70.) The chorda cells are found exclusively in 

 the anterior quadrants and in later stages they protrude to the 

 exterior along the injured side where they are found as scattered 

 cells in the perivitelline space. (Figs. 66-70.) In no case, save 

 one, have I seen any indication that these cells form a rod-shaped 

 notochord, and this case (Fig. 72) was that of a living embryo in 

 which it is possible that the notochord-like structure w^as really 

 composed of gastral endoderm and hence not a true notochord at 

 all. It is evident that the chorda cells are unable to give rise to a 



