A LIMEXTA R Y CA A^A L. 



02/) 



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

 is present in Aniphioxus 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, h, and in transverse 

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

 and 566) with the hypopharyngeal 

 groove of Aniphioxus and Ascidians. 

 It does not, however, long 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). Tliis opening is retained 

 throughout the Ammoccete condi- 

 tion, but the organ becomes highly 

 complicated, with paired anterior 

 and posterior horns and a median 

 spiral portion. In the adult the 

 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 body. 



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 eudostyle or hypopharyngeal groove. In all the higher Vertebrata 



Fig. 415. Diagrammatic trans- 

 verse SECTIONS THROUGH THE RRANCHIAI, 

 REGION OF YOUNG LARViE OF PeTROMY- 



zoN. (From Gegenbaur; after Calberla.) 

 d. branchial region of throat. 



Fig. 41G. Diagrammatic vketicai, section through the head of a larva of 



Petromyzon. 



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

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

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



c.h. cerebral hemisphere.; th. optic thalamus ; tH. infundibulum; j|j». pineal gland; 

 mb. mid-brain; cb. cerebellum; ind. medulla oblongata; au.v. auditory vesicle; op. 

 optic vesicle ; ol. olfactory pit ; ni. moath; br.c. branchial pouches; t/(. thyroid invo- 

 lution; v.ao. ventral aorta ; ht. ventricle of heart ; ch. notochord. 



P.. E. II. 40 



