PROCESS OF DEVELOPMENT IN AVIAN OVA. 521 



duct, the process of cleavage had progressed to a marked extent. 

 Large polygonal areas were only present at the margin. In 

 the central parts were small granular morphological elements, 

 loosely attached to one another, of which the upper ones were 

 riner, whilst those in the deeper layers were more coarsely 

 granular. In the peripheric parts the germ disk rested upon 

 the yolk, but in the central parts the two were separated from 

 each other by a flat cavity. A few morphological elements, 

 resembling those of the lowermost layer of the germ disk, lay 

 upon the floor of this cavity. 



Oviducal eggs of a still later stage presented little more that 

 was worthy of notice. The cleavage had progressed to a some- 

 what greater extent even at the marginal portions ; the mor- 

 phological elements were in general smaller ; the whole disk 

 was somewhat thinner, the cavity somewhat deeper, with a few- 

 larger, spherical, strongly granular formed elements resting on 

 its floor. Finally, in one oviducal egg a still more advanced 

 stage of development was met with. The cleavage elements, 

 or embryonal cells, as we may also term them, were partially 

 separable into two laminae, of which the upper (S, fig. 403) 

 was of closer texture, and was composed of smaller cells, whilst 

 the lower (Z)) contained somewhat larger and more coarsely 

 granular cells, so irregularly disposed that in some parts they 

 only formed a single tier, whilst in others they were seen in 

 section to be collected into heaps of from two to three cells 

 deep, presenting in consequence projections at these points. 



In freshly laid ova the germ has sometimes undergone no 

 development beyond that which has been already described, 

 whilst in other instances the lower layer, composed of larger 

 cells, is sharply separated from the upper throughout its whole 

 extent. 



As a general rule, it may be said that, in freshly laid eggs, 

 the separation of the germ into two layers is sometimes more, 

 sometimes less well marked, but that the cleavage is not as yet 

 quite completed. The morphological elements of the inferior 

 layer are still tolerably large, and some very large cleavage 

 spheroids (M, fig. 403) still occur, which project strongly down- 

 ward towards the cavity, sometimes even touching its floor, 

 so that the blastoderm appears in transverse section to be 



