626 MR W. E. AGAR ON DEVELOPMENT OF ANTERIOR MESODERM, AND 



into which they open, are so much distended at this stage as to fill up almost the whole 

 space between the coelomic lining and ectoderm, which space, in front of and behind this 

 region, is occupied by the ventral processes of the myotomes — see text figure 7. The 

 bulk of the pronephros is situated under Ml and M2, the anterior and posterior 



e.g.1___. 



e.g. 2 



e.g. 3 4 



e.g. 4 -1 



v.p.y.z. 1. __I 

 [cor. hy.] 



v.p. 2 



pect. f. ext 



v.p. 3 -""" 



,v.p. 4' 



Vjia— -a-prn.st 



•pm.d. 



p.prn.st 



Fig. 7. — Protopterus, stage circ. 29. Reconstruction of a thick horizontal slice by Graham Kerb's method. Only 

 the ectoderm, central nervous system, auditory vesicles, myotomes and pronephros are shown, a.pm.st., 

 anterior pronephrostome ; avd.ves., auditory vesicle; cor.hy., coraco-hyoid muscle; e.g. , 1, 2, etc., first, 

 second, etc., external gill ; pect.f.ext., external projection of pectoral fin ; p.prn.st., posterior pronephrostome ; 

 pm.d., pronephric duct ; v.p. y, z, 1, etc., the ventral processes of the corresponding myotomes. The ventral 

 end of v.p. 2 has broken off from the rest of M2 and is fused to the ends ofv.pp. 3 and 4. 



pronephrostomes being underneath these myotomes respectively. The pronephros thus 

 offers an obstruction to the outgrowth of the ventral process of Ml and M2, which 

 is avoided by the forward slope of the Ml process and the backward slope of the M2 

 process (fig. 7). The three myotomes in front of the pronephros (1, z, y,) give rise to 



