DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 



173 



tissue envelopes of the neural tube and the vertebral arches with 

 their ligaments are differentiated. 



Conditions similar to thoss of Selachians are also to be observed, 



Fig. 110. 



Fig. 111. 



Figs. 110 and 111. Diagrams of cross sections through younger and older Selachian embryos 

 to illustrate the development of the principal products of the middle germ-layer. After VAN 

 WIJHE, with some changes. 



Fig. 110. Cross section through the region of the pronephros of an embryo, in which the 

 myotomes (//<y) are in process of being constricted off. 



Fig. 111.- Cross section through a somewhat older embryo, in which the myotomes have just 

 been detached. 



nr, Neural tube; ch, chortla ; ao, aorta; sch, subnotochordal rod; mp, muscle-plate of the 

 primitive segment ; 10, zone of growth, at which the muscle-plate bends over into the cutis- 

 plate (cp) ; vb, portion connecting the primitive segment with the [walls of the] body-cavity, 

 out of which are developed, among other things, the mesonephric tubules uk (fig. Ill) ; 

 Kk, skeletogenous tissue, which arises as an outgrowth from the median wall of the con- 

 necting portion (rb) ; rn, pronephros ; mk l , parietal, mk", visceral middle layer, from the 

 walls of which mesenchyme is developed ; Ih, body-cavity ; Ik, entoderni ; h, cavity of the 

 primitive segment ; uk, mesonephric tubule, arisen from the connecting portion vb of the 

 diagram 110 ; uk 1 , place where the mesonephric tubule has detached itself from the primitive 

 segment ; v.g, mesonephric duct, with which the mesonephric tubule has united on the left 

 side; tr, union of the mesonephric tubule with the body-cavity (nephridial funnel) ; //;', 

 mes 2 , mesenchyme, which has arisen from the parietal and visceral lamellae of the middle 

 layer respectively. 



although less distinctly, in Reptiles, Birds, and Mammals ; they 

 have been described by REMAK, KOLLIKER, and others, and have been 

 brought into connection with the formation of the vertebral column. 

 The primitive segments, which are at first solid, soon acquire a 

 small cavity (fig. 116), around which the cells are arranged into a 



