868 



STRUCTURES FORMED FROM THE MESOBLAST. 



The recent observations of L. Gerlach and Strahl show that the chorda dorsalis is derived 

 from the hypoblast (tig. 663). It does not contain chondrin or glutin, but albumin (Eetzius). 



Protovertebr. The cells of the inesoblast, on each side of the chorda, 

 arrange themselves into cubical masses, always disposed in pairs behind each other, 

 the protovertebra (fig. 665, V and u). The first pair correspond to the atlas. 

 At a later period each protovertebra shows a marginal cellular area and a nuclear 

 area (fig. 665). Only part of it goes to form a future vertebra. The part of the 

 inesoblast lying external to the protovertebra?, the lateral plates (fig. 665, II, s), 

 splits into two layers, an upper one and a lower one, which, however, are united 

 by a median plate at the proto vertebrae. The space between the two layers of the 

 mesoblast is called the pleuro-peritoneal cavity, or the coelom of Haeckel (III, K). 

 The upper layer of the lateral plate becomes united to the epiblast, and forms 

 the cutaneo-muscular plate of German authors, or the somatopleure (fig. 665, 

 III, x\ tig. 667, so), while the inner one unites with the hypoblast to form the 

 intestinal plate of German authors, or the splanchnopleure (fig. 665, III, y ; fig. 

 667, sp). On the surfaces of these plates, which are directed towards each other, 

 the endothelium lining the pleuro-peritoneal cavity is developed. On the surface 



Fig. 667. 

 Transverse section of an embryo duck, am, amnion ; ao, aorta ; ca.v, cardinal vein ; ch, noto- 

 chord ; hy, hypoblast ; ?*, muscle-plate ; so, somatopleure ; sp, splanchnopleure ; sp.c, 

 spinal cord ; sy.ij, spinal ganglion ; st, segmental tube ; wd, Wolffian (segmental) duct. 



of the median plate, directed towards the coelom, some cylindrical cells, the 

 "germ-epithelium/' of Waldeyer, remain, which form the ovarian tubes and the 

 ova ( 438). 



According to Remak, the skin, the muscles of the trunk, and the blood-vessels, and according 

 to His, only the musculature of the trunk, are derived from the somatopleure. Both observers 

 agree that the splanchnopleure furnishes the musculature of the intestinal tract. 



Parablastic and Archiblastic Cells. According to His, the blood-vessels, 

 blood, and connective-tissue are not developed from true mesoblastic cells, but he 

 asserts that for this purpose certain cells wander in from the margins of the blasto- 

 derm between the epiblast and hypoblast, these cells being derived from outside the 

 position of the embryo, from the elements of the white yelk. His calls these struc- 

 tures parallactic, in opposition to the archiblastic, which belong to the three layers 

 of the embryo. Waldeyer also adheres to the parablastic structure of blood and 





