THE LAYERS OF THE BLASTODERM. 33 



by their proximity to the white yolk, whose spherules they 

 absorb. But my reason for thinking it probable that these cells 

 alone do not form the mesoblast are, ist. That the mesoblast 

 and hypoblast are formed nearly synchronously, and except at 

 the centre a fairly even sprinkling of lower layer cells is from 

 the first to be distinguished between the epiblast and hypoblast. 

 2nd. That if some of the lower layer cells are not converted into 

 mesoblast, it is difficult to see what becomes of them, since they 

 appear to be too numerous to be converted into the hypoblast 

 alone. 3rd. That the chief formation of mesoblast at first takes 

 place in the centre, while if the formative cells alone took part in 

 its formation, it would be natural to expect that it would begin 

 to be formed at the periphery. 



Oellacher himself has shewn (Zeitschrift fur wissenscliaftliche 

 Zoologie, 1873, " Beitrage zur Entwick. Gesch. der Knochen- 

 fische") that in osseous fishes the cells which break away from 

 the blastoderm take no share in the formation of the mesoblast, 

 so that we can derive no argument from the formation of the 

 mesoblast in these animals, for believing that in the chick it 

 is derived only from the formative cells. 



In the later stages, however, from the twelfth to the twenty- 

 fifth hour, the growth of the mesoblast depends almost entirely 

 on these cells, and Peremeschko's discovery of the fact is of 

 great value. 



Waldeyer (Henle tmd v. Pfeufer's Zeitschrift, xxxiv. Band, 

 fur 1869) has given a different account of the origin of the 

 layers. There is no doubt, however, in opposition to his state- 

 ments and drawings, that from the very first the hypoblast is 

 distinct from the mesoblast, which is, indeed, most conspicu- 

 ously shewn in good sections ; and his drawings of the deriva- 

 tion of the mesoblast from the epiblast are not very correct. 



The changes which have been described are also clearly 

 shewn by means of silver nitrate. Whereas, at first this reagent 

 brought out no outline markings of cells in the lower layer, 

 by the eighth to the twelfth hour the markings (PI. I, fig. 3) 

 are very plain, and shew that the hypoblast is a distinct coherent 

 membrane. 



In section, the cells of the hypoblast appear generally very 

 thin and spindle shaped, but the outlines brought out by the 



B. 3 



