EAELT STAGES OF DEVELOPMENT OF THE EABBTT. 159 



describes the appearance of a cavity between the outer layer of 

 epiblast and the inner layer of epiblast. It is hardly a cavity, 

 for it is partially filled by very ''stellate'' cells. With the 

 formation of this the inner layer of epiblast becomes arched 

 inwards, a process termed " temporary inversion of layer " by 

 Heape. Subsequently this arch flattens out again, and it and 

 the outer layer cells and stellate cells between them all fuse 

 together to form the permanent epiblast. 



I have not studied the mole, but from Heape's description 

 this seems to be an almost exact parallel to the process which 

 occurs in the rabbit, with this exception : whereas in the 

 rabbit the increase in activity of the inner layer of epiblast 

 gives rise to a rupture of the outer layer of epiblast, in the 

 mole one of the alternatives suggested above is taken, and the 

 inner epiblast bulges inwards, leaving a loose space threaded 

 across with " stellate " cells between it and the zona radiata. 

 Subsequently on the expansion of the whole blastodermic 

 vesicle the plate flattens out again and the " stellate " cells 

 and other outer layer cells become intermingled with those 

 of the plate and form the permanent epiblast. Heape 

 regards this as a kind of inversion, and the stellate cells as 

 " trager." 



I cannot agree with Heape in considering the stellate cells 

 he mentions as being equivalent to trager cells, and certainly I 

 do not think that the " Rauber cells " of the rabbit are in any 

 way connected with " trager " — but of this I shall say more in 

 a later paper. 



As regards the meaning of the fusion of the two layers, I do 

 not see that it need necessarily have any morphological signi- 

 ficance at all. It may be merely an accident of development. 

 At the same time I cannot entirely neglect certain occurrences 

 in another group of Vertebrates, and have discussed them in 

 another paper. 



By the union of the two layers the embryonic disc acquires 

 a very much more distinct outline, which is now practically 

 circular; its outline is considerably more regular than before 

 the junction just described has taken place. 



