AVES. 



149 



nar cells (ep). The lower or hypoblast segments are larger, in 

 some cases very much larger, than those of the epiblast, and are 



I 





FIG. 90. SECTION OF THE GERMINAL DISC OF A FOWL DURING THE LATER 



STAGES OF SEGMENTATION. 



The section, which represents rather more than half the breadth of the blastoderm 

 (the middle line being shewn at c), shews that the upper and central parts of the disc 

 segment faster than those below and towards the periphery. At the periphery the 

 segments are still very large. One of the larger segments is shewn at a. In the 

 majority of segments a nucleus can be seen; and it seems probable that the nucleus 

 is present in them all. Most of the segments are filled with highly refracting spherules, 

 but these are more numerous in some cells (especially the larger cells near the yolk) 

 than in others. In the central part of the blastoderm the upper cells have commenced 

 to form a distinct layer. No segmentation cavity is present. 



a. large peripheral cell ; b. larger cells of the lower parts of the blastoderm ; 

 c. middle line of blastoderm; e. edge of the blastoderm adjoining the white yolk ; 

 w. white yolk. 



so granular that their nuclei can only with difficulty be seen. 

 They form a somewhat irregular mass, several layers deep, and 

 thicker at the periphery than at the centre : they rest on a bed 

 of white yolk, from which they are in parts separated by a more 

 or less developed cavity, which is probably filled with fluid yolk 

 matter about to be absorbed. In the bed of white yolk nuclei 

 are present, which are of the same character, and have the same 

 general fate, as those in Elasmobranchii. They are generally 

 more numerous in the neighbourhood of the thickened periphery 

 of the blastoderm than elsewhere. Peculiar large spherical bodies 

 are to be found amongst the lower layer cells, which superficially 

 resemble the larger cells around them, and have been called 

 formative cells \vidc Foster and Balfour (No. 126)]. Their real 

 nature is still very doubtful, and though some are no doubt true 

 cells, others are perhaps only nutritive masses of yolk. In a 

 surface view the blastoderm, as the segmented germinal disc may 



