FORMATION OF THE LAYERS. 251 



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 : 



(1) The embryo swelling. 



(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 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 PL 7, fig. 2 a. An 

 average cell measures about -^ to -^ of an inch, but some of 

 the larger ones on the floor attain to the -^ 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 j^o f an mcn m 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. 



Though usually spherical the nuclei not infrequently have a 

 somewhat lobate form. 



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



172 



