74 BULLETIN OF THE 



safe to infer that he was wrong in supposing the larger accumulation of 

 cells shown in his figure to represent the caudal thickening rather than 

 the primitive cumulus. Fig. 39 is a transverse section through the 

 primitive cumulus in the region of its greater width. 



In radial sections of the egg during this stage the cells of the unmodi- 

 fied blastoderm appear lens-shaped, the deep surface being more convex 

 than the outer, and contain each a single large nucleus, that is usually 

 central in position (Fig. 40). They are frequently preserved in the pro- 

 cess of division, their nuclei exhibiting the customary dumb-bell shaped 

 figure (Figs. 42, 44). 



The " interzonal filaments " are quite persistent, remaining distinguish- 

 able even after the formation of the dividing cell wall (Fig. 44). 



The nuclei in nearly all the sections which were stained in borax car- 

 mine are in a condition very favorable for study. The filaments of 

 chromatine are deeply stained, the nucleoplasm only faintly. The ar- 

 rangement of the chromatic substance in the nuclei varies from a condi- 

 tion in which it is concentrated into a ball at the centre of the nucleus 

 (Fig. 43), to one in which it forms a hollow shell near the surface of the 

 latter. 



Sections during the protozonite stage show that the blastoderm of the 

 embryonic region consists of two distinct cell layers — the ectoderm and 

 the mesoderm (Figs. 49, 45). The cells of the outer layer (ectoderm) 

 are columnar (Fig. 45), and their nuclei, which are smaller than in pre- 

 vious stages, are very close together and much nearer the superficial 

 than the deep ends of the cells. The cells of the inner layer (meso- 

 derm) are not columnar but rounded cuboidal, and in general are much 

 less regularly arranged than the ectodermic cells; their nuclei, which 

 occupy the centres of the cells, do not at this stage present any other 

 characteristic differences from the nuclei of the ectoderm. At a later 

 period the nuclear elements of the mesoderm become spindle-shaped, and 

 thereby can be readily distinguished from those of the ectoderm. As 

 in the preceding stage, the cells of the non-embryonic or dorsal region 

 of the blastoderm are much flattened, even more than previously, and 

 only a single layer deep. 



The cellular elements of the mesoderm are not everywhere definitely 

 arranged, and the deep margin of the layer especially is irregular in out- 

 line ; it partly envelops the yolk corpuscles, which are reduced to small 

 fragments on the surfaces adjacent to the protoplasm, but it does not at 

 this time form an uninterrupted layer. 



The yolk corpuscles of this and succeeding stages are not absolutely 



