THE ORGANS OF THE MIDDLE GERM-LAYER. 



363 



^beginning a segmentally formed organ, as can be best followed in 

 the Selachians; for each mesonephric canal is developed in a single 

 segment. 



In Reptiles, Birds, and Mammals the connecting stalks are solid 



Fig. 204. 



Fig. 205. 



Figs. 204 and 205. Diagrams of cross sections through a younger and an older embryo Selachian 

 to show the development of the principal products of the middle germ-layer. After WIJHE, 

 with some alterations. 



Fig. 204. Cross section through the region of the pronephros of an embryo in which the muscle- 

 segments (mp) are in process of being constricted off. 



Fig. 205. Cross section through a somewhat older embryo, in which the muscle-segments have 

 just been constricted off. 



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

 primitive segment ; iv, zone of growth where the muscle-plate bends around into the cutis- 

 plate (cp) ; rb, the connecting piece which unites the primitive segment to the walls of the 

 body-cavity, and from which are developed, among other things, the mesonephric tubules 

 (fig. 205 uk) ; sk, skeletogenous tissue, which arises by a proliferation of the median wall 

 of the connecting piece vb ; vn, pronephros ; mk\ mk 2 , parietal and visceral middle layer, 

 out of which mesenchyma is developed ; Ih, body-cavity ; ik, entoblast ; h, cavity of the 

 primitive segment ; uk, mesonephric tubules, which have arisen from the connecting piece 

 vb of the diagram fig. 204 ; uk 1 , the place where the mesonephric tubule has been detached 

 from the primitive segment ; ug, mesonephric duct, with which, on the left side of the 

 figure, the mesonephric tubule has united ; tr, union of the mesonephric tubule with the 

 body-cavity (nephridial funnel) ; mes 1 , mes*, mesenchyma that has arisen from the parietal 

 and visceral middle layers. 



cords of cells (mesonephric cords). It is only when they have de- 

 tached themselves from the primitive segment, and their blind ends 

 have united with the mesonephric duct, that they acquire a small 

 cavity (fig. 202 si). Now they also become more readily distin- 

 guishable as separate canals, since they become farther removed from 



