142 s. HATTA. 



is ocrnpied by the yisceral Liver. There nre a few ee]]s lying" l)etweeii 

 tlie two Javer.s Avhicli oiia'lit perlin|)s to l)o lookcil ii|)(jii as l)o!oiio-ino- fo 

 the visceral laver. Tlie chorda -^f/iA////' is now distinct and separated 

 ivnui llie adjacent livpol^last hy pecnliar s]nndle-sliaped hypochordal 

 cells which are pro1)al)ly derived from cells originally lying lietween 

 Ihe chorda-hypoblast and the origin of the niesohlast. In fig. 17 

 from a still older embryo, the formati(m of the chorda is now complet- 

 ed, 'fhe hvpochordal cells surround it as before. The gut-hypoblast 

 lias slipped itself under the chorda and the hypocliordal cells, and 

 forms the definitive roof of the eriteiic cavity. Fig. 18 shows the 

 stage in which the medullary cord, the chorda, the hy))ochordal cells 

 and the mesoblast masses have been completely dilferentiated. 



It need scarcelv be pointed out that tlie process of the mesoblast 

 formation in the trunk is an al)l)re^ iated form of that observed in the 

 cephal<i-cervical region. ()ne f a-m doç.< not. of course, suddenly pass 

 into Ihe other, and in the anterior part of the trunk (at just about tlie 

 place where the head -protuberance is joined to the more j)osterior 

 o-lobular ])aft) W(^ can see tlie ])hases of gradual transition. 



V\<X. 2o is from the moi-e anterior ])art of this region. A' is tlie 

 f(_)ld tliat starts from the outei- wall of the enteric cavity to push itself 

 towards the median line. The gut-diverticulum is decidedly shallow- 

 er than in ha'. 20. Cells seem to l)e budding forth from the outer 

 extremity of its wall. Fig's. 24 and 25, fi'om an older embryo, are 

 sections taken more posteriorly. In fig. 24 the more anterior of the 

 two, the mesoblast mass is being laidded out from each side of the 

 chovd-d-anhige liut there is now no trace of the alimenta.ry diverticulum. 

 The hypochordal cells have slip]ied under the chorda. The cells of the 

 definitive gut-hv]ioblast are still outside the stri[) where the mesoblast 

 cells have lieen budded out. In lig. 25. they are pushing themselves 

 below the chorda and hyjioclKn-dal cells to firm the permanent median 



