ORIGIN OF THE MESODERM 



61 



Tnes. 



end. 



A little later a split at its dorsal end demarcates the mesoderm 

 rudiment from the notochord. The mesoderm rudiment, forming 

 now a broad band on each side of the embryo, becomes divided into 

 segments by splits which cut it across and a condition is reached 

 corresponding closely with that already described for Lepidosiren 

 where the mesoderm consists of a series of solid segments on each 

 side continuous veutrally with the mass of yolk cells forming the 

 main part of the endoderm. 



As in Lepidosiren the ventral unsegmented part of the mesoderm 

 becomes prolonged ventrally by the extension downwards of the split 

 between it and the endo- 

 derm. In other words the 

 mesoderm extends ven- 

 trally by a process of de- 

 lamination from the endo- 

 derm. 



In the anterior part of 

 the body the sheet of meso- 

 derm becomes split off 

 completely from the yolky 

 endoderm before it quite 

 reaches the mid - ventral 

 line so that the sheets be- 

 longing to the two sides 

 are discontinuous ventrally 

 but in the hinder region 

 the two splits meet ven- 

 trally so as to give rise to 

 a sheet of mesoderm con- 

 tinuous across the middle 

 line. Under ordinary cir- 

 cumstances the mesoderm 

 sheets in the anterior 

 region grow ventrally and 

 eventually fuse with one another (as will be described later) while 

 in the posterior region this fusion is anticipated by the two lateral 

 rudiments being continuous from the beginning. 



So far everything seems fairly simple, but it now remains to allude 

 to certain peculiarities which have done much to obscure the clear 

 understanding of the method of mesoderm formation and which are 

 especially important for the proper comprehension of the first forma- 

 tion of mesoderm in the meroblastic vertebrates. 



The peculiarities in question are to be seen in the hinder part of 

 the trunk region. In this region the split which separates off meso- 

 derm from endoderm remains for a time incomplete at a point just 

 external to the notochord. Each segment therefore remains for a 

 time continuous with the endoderm at this point. The level of these 

 junctions of mesoderm and endoderm is marked by a longitudinal 



Fig. 36. — Transverse section through an embryo of 

 Raua illustrating the origin of the mesoderm. 

 (After Sehwink, 1889.) 



ect, ectoderm ; end, endoderm ; nies, mesodmn ; 

 N, notochord. 



