THE LAYERS OF THE BLASTODERM. 31 



stages of segmentation. They do not differ from the general 

 lower layer cells except in size, and even pass into them by 

 insensible gradations. All the cells of the lower layer are 

 granular, and are filled with highly refracting spherules precisely 

 similar to the smaller white yolk spherules which line the bottom 

 of the segmentation cavity. 



The size of the ordinary cells of the lower layer varies 

 from 2Foo TFOO f an inch. The largest of the formative 

 cells come up to ^j of an inch. It will be seen from this 

 description that, morphologically speaking, we cannot attach 

 much importance to the formative cells. The fact that they 

 broke off from the blastoderm, towards the end of the seg- 

 mentation even if we accept it as a normal occurrence, rather 

 than the result of manipulation is not of much importance, and, 

 except in size, it is impossible to distinguish these cells from 

 other cells of the lower layer of the blastoderm. 



Physiologically, however, as will be afterwards shewn, they 

 are of considerable importance. 



The changes which the blastoderm undergoes during the 

 first three or four hours of incubation are not very noticeable. 

 At about the sixth or eighth hour, or in some cases consider- 

 ably earlier, changes begin to take place very rapidly. These 

 changes result in the formation of a hypoblast and mesoblast, 

 the upper layer of cells remaining comparatively unaltered 

 as the epiblast. 



To form the hypoblast a certain number of the cells of the 

 lower layer begin to undergo remarkable changes. From being 

 spherical and, as far as can be seen, non-nucleated, they become 

 (vide fig. 2 //) flattened and nucleated, still remaining granular, 

 but with fewer spherules. 



Here, then, is a direct change, of which all the stages can be 

 followed, of a cell of one kind into a cell of a totally different 

 character. The new cell is not formed by a destruction of 

 the old one, but directly from it by a process of metamorphosis. 

 These hypoblast cells are formed first at the centre and later 

 at the circumference, so that from the first the cells at the 

 circumference are less flattened and more granular than the 

 cells at the centre. A number of cells of the original lower 

 layer are enclosed between this layer and the epiblast ; and, 



