The lower layer cells form a mass rather than a layer, and constitute the whole of the blastoderm not included in the epiblast. The shape of this mass in a longitudinal section may be gathered from an examination of Plate 7, figs. 3 and 4.
It presents an especially thick portion forming the bulk of the embryonic swelling, and frequently contains one or two cavities, which from their constancy I regard as normal and not as artificial products.
In addition to the mass forming the embryonic swelling there is seen in sections another mass of lower layer cells at the opposite extremity of the blastoderm, connected with the former by a bridge of cells, which constitutes the roof of the segmentation cavity. The lower layer cells may thus be divided into three distinct parts:
(2) The thick rim of cells round the edge of the remainder of the blastoderm.
(3) The cells which form the roof of the segmentation cavity.
These three parts form a continuous whole, but in addition to these there exist the previously mentioned cells, which rest on the floor of the segmentation cavity.
With the exception of these latter, the lower layer is composed of cells having a fairly uniform size, and exhibits no trace of a division into two layers.
The cells are for the most part irregularly polygonal from mutual pressure; and in their shape and arrangement, exhibit a marked contrast to the epiblast cells. A few of the lower layer cells, highly magnified, are represented in Pl. 7, fig. 2_a_. An average cell measures about 1/800 to 1/900 of an inch, but some of the larger ones on the floor attain to the 1/475 of an inch.
Owing to my having had the good fortune to prepare some especially favourable specimens of this stage, it has been possible for me to make accurate observations both upon the nuclei of the cells of the blastoderm, and upon the nuclei of the yolk.
The nuclei of the blastoderm cells, both of the epiblast and lower layer, have a uniform structure. Those of the lower layer cells are about 1/1600 of an inch in diameter. Roughly speaking each consists of a spherical mass of clear protoplasm refracting more highly than the protoplasm of its cell. The nucleus appears in sections to be divided by deeply stained lines into a number of separate areas, and in each of these a deeply stained granule is placed. In some cases two or more of such granules may be seen in a single area. The whole of the nucleus stains with the colouring reagents more deeply than the protoplasm of the cells; but this is especially the case with the granules and lines.