€22 MR W. E. AGAR ON DEVELOPMENT OF ANTERIOR MESODERM, AND 



The probability suggests itself that the dilatator action of the dorsal part of the 

 muscle is put into use during inspiration, and that the ontogenetic change from a 

 simple (potential) constrictor to a compressor + dilatator is a true recapitulation of its 

 phylogenetic history. 



The foregoing description of the development of these muscles applies to Lepido- 

 siren. The gaps between the stages of Protopterus were too great to allow the 

 development to be followed with such detail. The later condition and innervation 

 are the same in the two genera. 



We have already seen good reason to believe that the anterior trunk myotomes 

 come to be dorsal to visceral arches belonging genetically to a region in front of them. 

 In the case of the constrictor pharyngis, which is also a structure belonging to the 

 vagus region, differential growth actually brings about the same result during 

 development. The facts are as follows : — 



In stage 30 (Lepidosiren) the posterior limit of the region of the pericardio- 

 peritoneal ducts from which the future muscle cells are being budded off is level with 

 the hind end of the vagus ganglion, and under myotome y, a large part of the region 

 being altogether in front of the occipital myotomes — i.e. in the palingenetic part of 

 the head. 



In stage 31 (Lepidosiren) the splanchnic portion of the constrictor reaches back to 

 the myoseptum between z and 1 (the first post-occipital myotome). 



In stage 38 (Lepidosiren) it goes back to at least myotome 5. 



In a Protopterus larva of 5*5 cm. it goes back to at least myotome 6. 



The ramus muscularis follows the extension of its muscle. 



The backward extension of the ventral part of the constrictor carries the dorsal 

 part back with it, though not so far, thus — 



At its first appearance (stage 31 + ) it is entirely confined to the region ventral to 

 the occipital myotome y (Lepidosiren). 



By stage 38 it stretches back as far as myotome 2 (Lepidosiren). 



In the 5 '5 cm. Protopterus it stretches back to myotome 3. 



The anterior ends of both parts of the constrictor remain approximately fixed. 



Differential growth also results in a shifting backwards relative to trunk myotomes 

 of the region of the alimentary canal supplied by the sensory branches of the vagus 

 (ramus intestinalis), as shown by the position of the glottis (which is supplied by this 

 nerve) at different stages. 



In stage 31 (Lepidosiren and Protopterus) the anterior boundary of the glottis 

 rudiment is opposite the posterior boundary of the vagus ganglion (under myotome y). 



In stage 34 (Lepidosiren) it is opposite myoseptum z—1. 



In stage 38 (Lepidosiren) it is opposite myotome 3. 



In the 5'5 cm. Protopterus it is opposite myotome 3. 



The outgrowth of the lungs from the glottis region of course carries the sensory 

 distribution of the vagus much further back still. 



