396 MARGARET ROBINSON. 



the beginnings of a second layer can be noticed. As one 

 follows the sections farther back one notices that the cells 

 gradually lose their cylindrical shape becoming flattened 

 (figs. 9 and 10). Sections through the strands show that all 

 along them mesoderm cells are apparently being budded off 

 from the blastoderm (figs. 9 and 10, vies.). In the anterior 

 region these cells are few and far between, but the farther 

 back we trace them the more numerous do they become, so 

 that on reaching the caudal thickening we find the strands to 

 consist of two definite layers of cells — ectoderm and mesoderm 

 (fig. 11). 



Besides the above-mentioned mesoderm cells there are 

 budded off from the blastoderm more cells of the type men- 

 tioned as occurring in the earlier stages (figs. 9 and 

 10). These cells have large much-vacuolated nuclei (about 

 twice as large as the nuclei of the other cells), and a 

 small amount of cytoplasm which is spread out in processes 

 resembling pseudopodia. In fact each cell has the appear- 

 ance of an amoeba with an immense nucleus. They are to be 

 found throughout the whole length of the embryo, but are 

 not numerous. In the embryo from Avhich the figured sec- 

 tions were taken there were about ten of these cells. Though 

 they come to lie in the yolk which is ultimately surrounded 

 by endoderm, these cells are distinctly mesodermal in origin, 

 i. e. they are budded off from the blastoderm in the same 

 manner, and in the same regions as the rest of the mesoderm. 

 It has been suggested by Kowalewsky (1886), and Nusbaum 

 (1886) that these cells help in some way to soften the yolk, 

 and so render it easy of absorption by the protoplasm. That 

 the yolk is of a different consistency in different stages can 

 be seen even in preserved specimens, and in the later stages 

 the vitellophags are first diminished in numbers and then 

 gradually disappear. These facts taken together certainly 

 support the above-mentioned view as to the function of these 

 cells. 



Sections through the posterior end of the embryo pass 

 through the hind ends of the lateral thickenings as well as 



