320 Prof. J. T. Wilson and Mr. J. P. Hill. [Jan. 21, 



matically in fig. 2. More adequate illustration by photomicrography 

 and otherwise will be forthcoming in a future communication. 



Fig. 2. 



In mesial section, the knot is seen to form a thick and prominent 

 lenticular mass projecting into the cavity of the blastodermic vesicle. 

 It is largely composed of a loose reticular tissue in which nuclei are 

 only sparsely distributed and cell outlines are for the most part 

 invisible. This tissue is thickly dotted with minute yolk-spherules 

 and small vacuoles, and is not limited towards the cavity of the 

 vesicle by any very sharp or clear-cut boundary. This reticulum of 

 the knot is continuous peripherally with the yolk-entoderm of the 

 bilaminar blastoderm around the knot. 



Penetrating the interior of the knot is the archenteric or gastrula- 

 cavity, opening on the surface at the blastoporic aperture near the 

 hinder part of the knot and appearing in sagittal section as a curved 

 canal passing from the blastopore at first deeply, and then forwards, 

 to end blindly in the more anterior part of the knot. This cavity is 

 lined throughout by a very definite cellular wall. 



Both in front of and behind the knot, the blastoderm is simply bila- 

 minar, with thin ectoderm closely applied to the deep surface of the 

 vitelline membrane. The entodermal cells are large and contain yolk- 

 spherules of varying size and staining reaction and loose yolk-spheres 

 are also found adherent to its deep surface. 



The thin ectoderm . is continued over the knot from the region in 



