THE PROGTOBjEUM. 641 



In Birds the formation of the proctodseum is somewhat more compli- 

 cated than in other types, owing to the outgrowth from it of the bursa 

 Fabricii. 



The proctodfeum first appears when the folding off of the tail end of 

 the embryo commences (fig. 429, a?^) and is placed near the front (originally 

 the apparent hind) end of the primitive streak. Its position marks out 

 the front border of the postanal section of the gut. 



The bursa Fabricii first appears on the seventh day (in the chick), as a 

 dorsal outgrowth of the proctodseum. The actual perforation of the sej> 

 tum between the proctod^eum and the cloacal section of the alimentary tract 

 is not effected till about the fifteenth daj^ of foetal life, and the approxi- 

 mation of the epithelial layers of the two organs, preparatory to their 

 absorption, is partly effected by the tunneling of the mesoblastic tissue 

 between them by numerous spaces. 



The hypoblastic section of the cloaca of birds, which receives the open- 

 ings of the urinogenital ducts, is permanently marked off by a fold fi-om 

 the epiblastic section or true proctodaeum, with which the bursa Fabricii 

 communicates. 



Bibliography. 

 Alimentary Canal and its appendages. 



(561) B. Afauassiew. "Ueber Bau u. Entwicklung d. Thymus d. Saugeth." 

 Archiv f. viikr. Anat. Bd. xiv, 1877. 



(562) Fr. Boll. Das Princip d. Waclisthums. Berliu, 1876. 



(563) E. Gasser. "Die Entstehung d. Cloakenoffnung bei Hiihnerembryonen." 

 Archiv f. Anat. u. Plnjsiol., Anat. Abth. 1880. 



(564) A. Gotte. Beitracie zur Entwicklungsgeschichted. Dartnkanals imHuhnchen. 

 1867. 



(565) W. Miiller. " Ueber die Entwickelung der Schilddriise. " Jenaische Zeit- 

 schrift, Vol. yi. 1871. 



(■i'6) W. Miiller. "Die Hypobranchiahinne d. Tunicaten." Jenaische Zeit' 

 schrift, Vol. vir. 1872. 



(567) S. L. Schenk. "Die Bauchspeicheldruse d. Embryo." Anatomisch-phy- 

 siologische JJntersuchungen. 1872. 



(568) E. Selenka. "Beitrag zur Entwicklungsgeschichte d. Luftsacke d. Huhna." 

 Zeit.f. wiss, Zool. 1866. 



(569) L. Stieda. Untersuch. ilh. d. Entwick. d. Glandtda Thymus, Glandula 

 thyroidea, u. Glandula carotica. 'Leipzig, 1881. 



(570) C. Fr. Wolff. "De foi-matione intestinorum. " Nov. Comment. Akad. 

 Petrop. 1766. 



(571) A. Wolfler. Ueb. d. Enticick. u. d. Bau d. Schilddriise. Berlin, 1880. 

 Vide also Kolliker (298), Gotte (296), His (232 and 297), Foster and Balfour (295), 



Balfour (292), Eemak (302), Schenk (303), etc. 



Teeth. 



(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of 

 Micros. Science, Vol. iii. 1855. 



(573) E. Owen. Odontography. London, 1840 — 1845. 



(574) Ch. S. Tomes. 3Ianual of dental anatomy, human and comparative. London, 

 1876. 



(575) Ch. S. Tomes. "On the development of teeth." Quart. J. of Micros. 

 Science, Vol. xvi. 1876. 



(576) W. Waldeyer. "Structure and development of teeth." Stricker^s His- 

 tology. 1870. 



Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc. 



B. E. II. 41 



