MORPHOLOGY OF CEANIAL :\[USCLES IN SOME YEirrEBRATES. 261 



laryngei s. dorso-traclieales were serially liomologoiis with 

 the levatores arcuuiu branchialiuin (called by him " dorso- 

 branchiales "), that the laryngei were lioinologons with the 

 transversi ventrales (called by him " pharyngo-branchiales "), 

 and that these muscles and the laryngeal cartilages belonged 

 to the fifth branchial arch. 



This theory of the nature of the laryngeal muscles was 

 formed on the evidence of adult anatomy only, without 

 knowledge of the method of development. 



Wiedersheim stated that the fibro-cartilage pi-esent in 

 association with the ventral larynx of Protopterus might be 

 due to the remains of a sixth arch, " noch plausibler aber 

 erscheint es mir die betreffenden Knorpel auf einen in der 

 Raphe des pharyngealen Constrictors sich abspielenden 

 Chondrificationsprocess, das heisst auf Muskelwirknng 

 ziiruckznfuhren " ; also that "a priori steht der Annahme 

 gewiss nichts in der Wege das sich audi bei Amphibien die 

 primitive Cartilago lateralis als eine Sehnenverknorpelung 

 bilden kann oline das dabei phylogenetische Beziehung zu 

 den Kiemenbogeu angenommen warden mussen." 



In Necturus the larynx is formed as a median ventral 

 pouch of the pharynx in the hinder part of tJie fourth 

 branchial segment in lo mm. embryos (Text-figs. 53, 54). The 

 cells which form the constrictor of the oesophagus are budded 

 off from a thickened portion of the splanchnic coelomic 

 epithelium, and spread upwards round it (Text-fig. 54). A 

 similar production of splanchnic mesoblast from a tliickened 

 portion of splanchnic coelomic epithelium takes place 

 in the fourth branchial segment (Text-fig. 52), and the 

 cells spread upwards round the developing larynx and 

 pharynx. In 144 mm. embryos this splanchnic meso- 

 derm spreads upwards on either side of the pharynx 

 internal to the fourth levatores arcuum branchialium, and 

 forms the Aulagen of the dorso-laryngei and of the pharyn- 

 geal constrictor (Text-fig. 56). The splanchnic mesoderm 

 Tentral to the pharynx develops into the laryngei muscles, 

 and, next the larynx, the laryngeal cartilages. 



