510 SUMMARY OF CURRENT RESEARCHES RELATING TO 



structures in the toad (Bufo lentiginosus). The passage is in the main 

 derived from the dorsalmost portion of the hyomandibular fold (cleft) > 

 but its history, up to the time when it acquires a lumen and unites with 

 an outgrowth from the pharynx, is very complex. Its final position 

 between the mandibular and hyoid bars is produced by the separation of 

 the latter from the quadrate and its attachment to the auditory capsule 

 posterior to the tube. The annular cartilage arises at a stage immedi- 

 ately preceding the protrusion of the fore-limbs. Its primordium forms 

 a dense cellular strand derived from the perichondrium of the quadrate 

 and surrounding the tympanic portion of the tubal primordium. The 

 stapes arises within the membrane closing the fenestra ovalis, and has 

 no connection with any of the visceral arches. The columella auris is 

 first met with in the early stages of the metamorphosis, as a compact 

 cellular strand extending forward from the stapes and terminating 

 imperceptibly in the connective-tissue. It continues to grow forward, 

 and acquires connection w T ith the quadrate. Continued growth brings it 

 into contact with the tympanic cavity. Chondrification begins in the 

 posterior portion of the rod. But we are not able to indicate more than 

 a moiety of the author's results. 



Symmetrical Double Monstrosities in the Trout.* — Dr. J. F. Gem- 

 mill has studied a series of four-month old monstrosities, and dis- 

 tinguishes four types : — (1) United in head region, (a) the twin brains 

 united at the mesencephalon, (b) at the medulla oblongata ; ("2) united 

 in pectoral region, (a) with pectoral fins absent on adjacent side, 

 (b~) present but united ; (3) united behind the pectoral region, («) at 

 a considerable distance in front of the vent, (b) close to the vent ; 

 (4) united by the yolk-sac only. 



At the region of transition in laterally symmetrical double mons- 

 trosities, the notochords are the last structures to unite ; the equally 

 primitive neural axis and gut lose their duplicity earlier ; those parts of 

 the neural axis and gut which are most closely apposed to the notochord 

 retain evidence of original duplicity longer than parts that are more 

 remote. Duplicity of the dorsal aorta, of the pronephric glomerulus, 

 of the vertebral cartilages, of the body muscles, and of various other 

 structures, is correlated with duplicity of the notochord. In paired 

 organs the transition from the double to the single condition takes 

 place at the expense of the inner or adjacent elements, which unite and 

 are reduced. It is inferred that fusion has played a not unimportant 

 part in moulding the form of the neural axis and the alimentary tract in 

 the transition region. The law that union takes place between homo- 

 logous structures always holds good. With the rarest exceptions, 

 double monstrosities in the trout are examples either of anterior duplicity 

 or of union by the yolk-sac only, in marked contrast to what occurs in 

 birds and mammals. An explanation is suggested which depends on the 

 mode of origin of the primitive streak and the overgrowth of the blasto- 

 derm on the yolk mass. 



Development of Ceratodus.f — Dr. Gregg Wilson finds that the lung 

 of Ceratodus arises as a pit or depression on the floor of the pharynx just 



* Proc. Roy. Soc, lxviii. (1901) pp. 129-34. 



t Proc. Hoy. Pbys. Soe. Edin., xiv. (1901) pp. 319-23 (3 figs.). 



