ON THE FORMATION OF THE OERMTNAT. LAYERS TN PETROMYZON. 141 



oonstituent elements have acquired ilie t:ill coliimnm* shape liv repeated 

 lonoitndinal di\i8ion. Tlie liypohlastic cells iniiiiediatelv outside the 

 r'li(>rda-h\'pohlast have an elono-ated sliape and are in active nmltiplic;! - 

 lion as sho\vn hy many karyokinetic figares seen in this ]>arr. Tlie 

 products of the divisions are accumulated as a triano'ular mass lietween 

 the epihlast and tlie yolk. This is the jne>iohIa^f (/Hi-'.s.). It is clearly 

 distinguished from the yolk mass not onlv hv a distinct slit hut also 

 hy the characters of its coiistituent elements whi('h are smalh^r. liave 

 their nuclei stained deeper, and are less full of ^•olk granules than volk- 

 cells. Also karyokinetic figures are often visi])k' in tliis mass, while 

 they are of rare «x'currence in tlie yolk-cells. Tlius. the mesohlast 

 arises liere as paired masses liudded out from a few ]iypol)last cells 

 placed immediately outside tl\e chorda-hypo1)last. This account 

 agrees suhstantially with that given hy Calberla.' The mesohlast cells 

 facing the e|)ihl:ist acquire a cohminar shape, arrange themselves more 

 or less regularly, and l)ecome tlie parietal laver of the mesol)last. In 

 fig. 13. tliis layer has l)een more or less formed and has l)ecome c()u- 

 tinuous with tlie external extremity of the chorda-hvpohlnst. The 

 cells lying helow the parietal .layer are irregularly ari-;inged and are 

 still having new cells added from tlie liypohlast cells of the aforesaid 

 part. 



Figs. 1 4 and 1 5 are from th(î dors;d region of slight! v older eml)rvos. 

 Fig. 14 is essentially like fig. 13. \n fig. 15, which represents a sec- 

 tion more posterior than fig. 14, the mesohlast mass of one side is isolat- 

 ed, while that of the other side is still continuous with the hy[)oblast. In 

 fig. 16, the mesol)last masses of both sides are entirelv cut otf from 

 the hypoblast and have assumed a trianguhu- sha.pe, of which the 

 inner and outer sides are formed by the parietal, while the ventral side 



1. E. Calberla : — Uobei' die Eutwieklunt;- d. McdullarroliTcs ii. d. I'horda dursalis d. 'IV'lcosI ier 

 u, d. Petromyzonton. INForph. .Tahrb. Bd. III. 



