ALIMENTARY CANAL. 



625 



organ secreting mucus. As was first pointed out by W. Miiller there 

 is present in Amphioxus a very similar and probably homologous 

 organ, known as the hypopharyngeal groove. 



In the higher Vertebrata this organ never retains its primitive 

 condition in the adult state. In the larva of Petromyzon there is, 

 however, present a ventral groove-like diverticulum of the throat, 

 extending from about the second to the fourth visceral cleft. This 

 organ is shewn in longitudinal section in fig. 414,/;,, and in transverse 

 section in fig. 415, and has been identified by W. Miiller (Nos. 565 

 and 566) with the hypopharyngeal 

 groove of Amphioxus and Ascidians. 

 It does not, however, lung retain its 

 primitive condition, but its open- 

 ing becomes gradually reduced to a 

 pore, placed between the third and 

 fourth of the permanent clefts (fig. 

 416, th}. This opening is retained 

 throughout the Amrnocoete condi- 

 tion, but the organ becomes highly 

 complicated, with paired anterior 



and posterior horns and a median ZO N. (From Gegenbaur ; after Calberla. 

 spiral portion. ILL the adult the d. branchial regiou of throat, 



connection with the pharynx is ob- 

 literated, and the organ is partly absorbed .and partly divided up into 

 a series of glandular follicles, and eventually forms the thyroid boiJi/. 



From the consideration of the above facts W. Miiller was led to 

 the conclusion that the thyroid body of the Craniata was derived from 

 the endostyle or hypopharyngeal groove. In all the higher Vertebrata 



TRANS- 



FIG. 41o. DIAGRAMMATIC 



VERSE SECTIONS THROUGH THE BRANCHIAL 

 REGION OF YOUNG LARVAE OP PETROMY- 



fftt.v 



me/ 



ch _ 



FIG. 41(5. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA OF 



PKTROMYZOX. 



The larva had been hatched three days, and was 4-8 mm. in length. The optic and 

 auditory vesicles are supposed to be seen through the tissues. The letter tr pointing 

 to the base of the velum is where Scott believes the hyomaudibular cleft to be situated. 



c.h. cerebral hemisphere ; //;. optic thalamus; -in. infumlibulum; pn. pineal gland ; 

 nib. mid-brain; cb. cerebellum; mil. medulla oblougata; aii.r. auditory vesicle; op- 

 optic vesicle ; ol. olfactory pit; in. mouth; br.c. branchial pouches; Hi. thyroidinvo 

 liitiim ; v.ao. ventral aorta ; tit. ventricle of heart ; ch. notochord. 



r,. E. n. 



