Tin: <!:<; LMfl OF 'nil-: Mii'M i \YI:::. 



36S 



forim-d ori/an. M ran !> h-t i'ollmvcd in 

 tin- Si-larhians ; for rach metonepbric canal is developed in a tingle 

 Begment. 



In U..'ili--. IlinK :ii!l M:iliiln:ils tin- coniurl ini; stalks an> solid 



Fig. 204. 



Pig. 205. 



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

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

 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 ; rA, chorda ; ao, aorta ; tch, subnotochordal rod ; mp, muscle-plate of the 

 primitive segment ; ?r, zone of growth where the muscle-plate bends around into the 

 plate (<;) ; vb, the connecting piece whicli unites the primitive segment to the walls of the 

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

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

 of the connecting piece vb ; r, pronephros ; mk\ ink*, parietal and visceral middle layer, 

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

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

 t-6 of the diagram fig. '204 ; uk 1 , the place where the mesonephric tubule has been det 

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

 figure, the mesonephrir tulmle has united ; t,-, union of the mesonephric tubule with the 

 body-cavity (nephridial funnel); ;,;', met*, mesenchyma that has arisen from the ]> 

 and visceral middle layers. 



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

 taclu-d tlit-iiix-lv. s from tin- pi-iinitive segment, and their blind ends 

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

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

 as srparato canals, since tln-y hi'cdine farther removed from 



