26 MESENCHYME THEORY OF HERTWIG. 



late years it has obtained, in its modified form as above given, the adherence of m^ny 

 embryologists, and especially of R. and O. Hertwig, Kupffer, Kollmann, and Waldeyer. R. 

 and 0. Hertwig have given the name of metenchyme to His's parablast, while retaining the 

 designation of middle germinal layer or mesoderm for the rest of the mesoblast, from which 

 it differs (1) in its structure, consisting of loosely arranged wandering cells, as distinguished 

 from the epithelium -like larrellse. of which according to their description the rest of the 

 mesoderm is composed ; (2) in its derivation, arising as separate cells from the entoderm instead 

 of in the form of a coherent layer ; and (3) in its further development and destination, giving 

 origin to the connective tissues and blood-vessels, and perhaps to the plain muscular tissue, 

 whereas the mesoderm proper gives origin to the skeletal muscles and to the epithelium of the 

 S31OUS cavities, and of the genital and urinary organs. They describe the true mesoderm as 

 consisting of two epithelial lamellas, which form distinct layers of the blastoderm, so that 

 according to this view the complete blastoderm would consist of four layers (epiblast. outer 

 or somatic mesoblast, inner or splanchnic mesoblast and hypoblast) besides the mesenchyme : 

 to which mutt be added a median strand of cells set aside for the formation of the notochord. 

 It appears evident from the researches of Kowalevsky in Sagitta and Amphioxus that what 

 is to be regarded as the typical origin of the mesoblast in Metazoa 1 takes the form of a pair of 

 diverticula from the primitive archenteric cavity (fig. 28, la ; fig. 29, I), which hollow diverti- 

 cula become pinched off from the remainder of that cavity, and. their cavities becoming com- 

 pressed laterally, are converted into the ccelom or body-cavity (serous cavities of vertebrata), the 

 two walls of this cavity on either side forming respectively the inner and outer mesodermic 

 layers of R. and 0. Hertwig. In Sagitta, the diverticula occur in the neighbourhood of the 



I. 



Fig. 28. FORMATION OP MESOBLASTIC SOMITES IN AMPHIOXUS, SHOWN IN LONGITUDINAL OPTICAL 



SECTION. (Hatschek. ) 



la., dorsal view of an embryo in which the mesoblast is beginning to form as two longitudinal folds 

 of the hypoblast which are becoming subdivided from before back by constrictions into separate 

 somites. I., the same viewed in profile, showing the anteiior three somites of one side, with their 

 cavities in free communication with the enteric cavity. The neural canal, n.c., is continued 

 posteriorly by a neurenteric canal into the enteric cavity ; cp, epiblast ; hy, hypoblast. II., dorsal 

 view of a more advanced embryo. The somites are more numerous and are completely separate. In 

 all but the most anterior pair the communication with the enteric cavity is still seen. III., dorsal 

 view at a still later stage. The somite cavities are now completely closed. The cellular rod, ch, shown 

 running along the middle of the embryo is the notochord. 



blastopore. which is also the typical seat of origin of the mesoblast, but in Amphioxus they 

 are formed by longitudinal folds of the wall of the archenteric cavity, which grow from 

 before backwards, and become separated up into segments in their progress. In most vertebrates 

 above Amphioxus the mesoblastic outgrowths are from the first solid, not hollow (although a 

 split may early, and does eventually in any case, occur in them, the ccelom being thus pro- 

 duced), nor do they originate so distinctly from the entoderm, but arise rather at the junction 



1 Except the Coelenterata which have only the two primary layers. 



