38 FORMATION OF THE LAYERS. 



These three parts form a continuous whole, but in addi- 

 tion 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 com- 

 posed 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 PI. III. fig. 

 2 a. An average cell measures about gi^^ to ^i^^ of an inch, 

 but some of the larger ones on the floor attain to the ^j^ 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, liave a uniform structure. Those of the lower 

 layer cells are about j^L_ of an inch in diameter. Roughly 

 speaking each con.sists 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. 



Though usually spherical the nuclei not infrequently have 

 a somewhat lobate form. 



Very similar to these nuclei are the nuclei of the yolk. 

 One of the most important differences between the two is 

 that of size. The majority of the nuclei present in the yolk are 

 as large or larger than an ordinary blastoderm cell; while many 

 of them reach a size very much greater than this. The ex- 

 amples I have measured varied from ^J^ to 230 ^^ ^"^ i^<^^^ i^^ 

 diameter. 



