208 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



have found ; Fig. 5 is a little older, and Fig. 7 is still older, and is part of 

 the section which is figured in Plate XXXV. The details of the histo- 

 logical structure of the germinal mass are shown in Plate XLI, Fig. 7, 

 which is a portion of Plate XX, Fig. 5, more highly magnified. 



The youngest embryo in which I have been able to find the germinal 

 mass was only a little older than the one shown in Plate XLI, Fig. 2, 

 and in Plate XVIII, although I failed to find it in this embryo. 



As shown by Plate XLI, Fig. 7, and by the figures on Plate XX, the 

 granular protoplasm between the closely packed nuclei is divided up into 

 cells, although the cell boundaries are only faintly visible here and there. 

 It will be seen from Plate XLI, Fig. 7, that the nuclei of the germinal 

 mass n do not differ in any marked way, except in the activity with 

 which they multiply, from the embryonic cells of the surrounding ecto- 

 derm a, and endoderm 6, although they are quite different from the 

 follicle cells, of which one is shown at B, and also from the blood cor- 

 puscles &/, and mesenchyma cells A. As Figs. 5 and 6 of Plate XX show, 

 the eleoblast of the embryo consists, for the most part, of two sorts of 

 cells, small, granular, amoeboid blood corpuscles and mesenchyma cells, 

 and large, vacuolated, migratory, follicle cells, in process of degeneration. 

 These cells are all so different from those which make up the germinal 

 mass, that there is a strong presumption against its derivation from 

 either of them, while the resemblance between its cells and those of the 

 ectoderm and endoderm, and the activity with which they multiply, are 

 reasons for believing that it arises from blastomeres, like those which 

 give rise to the germ layers of the embryo, and possibly from the division 

 of a single blastomere. The most noteworthy peculiarities of the germ- 

 inal mass at the stage shown in Plate XLI, Fig. 7, are, first, that it is 

 homogeneous, or without differentiation among its cells, and secondly, 

 that cells in all parts of it are multiplying by karyokinesis. 



The germinal mass is present as a definite, distinctly limited struc- 

 ture before the stolon is formed, but as this becomes developed, the 

 germinal mass becomes folded into it, as is shown in the figures, and it 

 also becomes elongated into a rod which runs along the hgemal side of 

 the stolon, as shown at n in Plate XLI, Figs. 4 and 6, and in longitudinal 

 section in Plate XVI, Fig. 5, and in transverse section of a young stolon in 

 Plate XXI ; and also in Plate XXXIV, Fig. 1, which is from the proximal 

 end or root of a fully grown stolon. 



As it lengthens its cells become differentiated and specialized in a 

 manner which will soon be described, and all recent writers agree that it 



