1,1.', i LOPM1 ONH Tl\ I -I I ,M> HLOOD. 



173 



tissue envelopes of the neural tube and the vertebral arches with 

 their ligaments uro ditl'ercnt i 



Conditions similar to those 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 



\VI.HIK, with some changes. 



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

 myotomes (n<p) 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, Neuial tube; ch, chorda: ao, aorta; gch, subnotochordal rod; mp, muscle plate of the 

 primitive segment ; ?r 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, 

 nut of which arc developed, amoni; other things, the inesonephric tubules uk (fig. Ill) ; 

 8k, skeletogenous tissue, which arises as an outgrowth from the median wall of the con- 

 nect ing ix.rtion (cU): m, pronephros; ink 1 , parietal, mk', visceral middle layer, from the 

 walls of which me.-eiiehyme is developed ; Ih, body-cavity ; ik, entoderm ; h, cavity of the 

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

 diagram 110 ; uk l , place where the mesonephric tubule has deach<-l it vlf from the primitive 

 n1 ; c.'/, niex'iiephiic duct, \\ith which the iiu'-tnu-phric tubule has united on the left 

 bide: tr, union of the mes mephi a- tubule with the body-cavity (nephridial funnel) ; mes 1 , 

 met', roesenchyme. which has arisen from the parietal and visceral lamellae of the middle 

 la\ IT : 



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

 have been described by REMAK, KHI.I.IKKR, and others, and have been 

 brought into connection with the formation of the vertebral column. 

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

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



