288 PROCEEDINGS OP THE AMERICAN ACADEMY. 



which takes place before the fourteenth day, and has been well figured 

 by Keibel ('93) ; this growth undoubtedly takes place very rapidly, 

 for Bonnet ('91) has estimated that in the sheep, where a similar 

 though less extensive growth occurs, the embryo must elongate at the 

 rate of more than 1 cm. per hour, and that in the pig the growth is 

 still more rapid. The germinal disk varies in size from 0.1 mm. in 

 the smallest to 0.265 mm. in the largest embryo here considered. 



In describing the embryos more at length 1 shall speak of the layer 

 of nearly isodiametric cells as the ectoderm, and of the inner layer of 

 flattened cells as the entoderm ; the relation of these to the outer dis- 

 integrating layer, which with Rauber ('75) is best designated as the 

 " Deckschicht," I shall discuss later. The two prominent layers 

 (ectoderm and entoderm) are distinguished from each other not only 

 by the shape of their component cells, but also by the shape of their 

 nuclei, and by the chromatic reaction of their protoplasm. The nu- 

 clei of the extra-germinal region of the ectoderm are nearly always 

 perfectly spherical ; those of the germinal disk, especially in later 

 stages of development, are more often slightly elongated or ellipsoid, 

 with the longest axis at right angles to the surface of the disk ; 

 there are significant exceptions to this last rule which will be con- 

 sidered later. The entodermal nuclei in the extra-germinal region 

 are generally flattened parallel to the surface of the embryonic vesicle ; 

 those in the region of the disk also have this shape in the younger 

 embryos, but in the older stages they are spherical, and the cells 

 in which they lie are essentially isodiametric. Furthermore, when 

 properly decolorized the hgematoxylin gives three distinct shades of 

 blue to the embryonic cytoplasm. The extra-germinal ectoderm is 

 stained a lip-ht blue, the ectoderm of the germinal disk is sharply 

 marked off from this by a deeper shade, and the cytoplasm of the ento- 

 dermal cells stains uniformly a still deeper blue. This diiferentiation 

 in color, which is often of very great value, does not appear before the 

 embryo is decolorized, and it disappears if the process of decolorizing 

 is carried too far, for in that case the whole vesicle becomes a uniform 

 light blue. In almost every instance, these three shades of blue are 

 manifest in my preparations. The chromatic substance of the nucleus 

 stains deeply throughout the vesicle, both in resting nuclei and in those 

 undergoing karyokinetic changes. In the latter, when cut at the 

 proper angle, the nuclear spindle and the protoplasmic radiations 

 around the centrosomes can be clearly seen. It should further be 

 noted, that this stain brings out the cell walls with great distinctness, 

 especially in the ectoderm. In the entoderm the cells are extremely 



