TWENTY-FOUBTH ANNUAL MEETING. 15 



uous in the embryo face. Their failure to unite is the cause of that embryological 

 deformity known as harelip. 



Of the origin of the vertebrate mouth. Mr. Bettany writes (following Doctor 

 Dohrn, "Nature,"') that '"the vertebrate mouth is a modern structure, and arises ex- 

 traordinarily late in development. The embryonic body is almost completely formed, 

 all the great systems are established, while as yet there is no mouth. It does not 

 arise either in the position in which it permanently remains in the majority of ver- 

 tebrates, but undergoes considerable shifting. Only in some fishes does it retain its 

 primitive situation. The study of its development establishes the idea that the 

 mouth of vertebrates is homologous with the gill clefts." 



Haeokel, in "History of Creation," again says: "In ail craniota, that is. all verte- 

 brate animals having a skull and brain, the brain, which is at first only the bladder- 

 shaped dilations of the spinal marrow, divides into five bladders, lying one behind 

 the other. This is just the same in all vertebrate embryos, from the lowest to the 

 highest. The whole form of the body is as yet exceedingly simple, being merely a 

 thin, leaf-like disk. Face, legs, intestines, etc., are completely wanting, but the five 

 bladders are quite distinct. The first bladder, the fore brain, develops into the 

 hemispheres of the large brain, the cerebrum. The second forms the center of sight," 

 etc. . . . "An exceedingly important formation, of which we are not able to 

 recognize a trace in the full-grown animal, are the gill arches, which originally are 

 common to all vertebrate animals, but which at a later period are transformed into 

 the most different organs. Everyone knows the gill arches of fish — those arched 

 bones which lie behind one another to the number of three or four, on each side of 

 the neck, and which support the gills, the respiratory organs of the fish. Now, these 

 gill arches originally exist in embryo exactly the same in man, in dogs, in fowls, in 

 tortoises, as well as in other vertebrate animals. It is only in fishes that these re- 

 main in their original form and develop into respiratory organs. In the other ver- 

 tebrate animals they are partly employed in the formation of the face (especially 

 the jaw apparatus) and partly in the formation of the organ of hearing. 



"An examination of the human embryo in the third or fourth week of its exist- 

 ence shows it to be altogether different from the fully-developed man, and that it 

 exactly corresponds to the undeveloped embryo form presented by the ape, dog, 

 rabbit, or other mammals at the same stage of their ontogeny. At this stage it is a 

 bean-shaped body of very simple structure, with a tail behind and two pairs of 

 paddles, resembling the fins of a fish. Nearly the whole of the front half of the 

 body consists of a shapeless bud without a face, on the sides of which are several 

 gill fissures and gill arches, as in fishes. ... In the embryo of man, as in all 

 other vertebrates, the very remarkable and important structures which are called 

 the gill arches and gill openings appear at a very early period, on each side of the 

 head. These are among the characteristic and never-failing organs of vertebrates. 

 . . . The number of these gill arches and of the gill openings between amounts, 

 in the higher vertebrates, to four or five on each side; but the lower forms have a 

 yet larger number. Originally they are used for respiration. In the higher verte- 

 brates they afterwards close, and are transformed partly into the jaws and partly 

 into the bonelets of the ear. Almost simultaneously with development of the gill 

 arches, and immediately behind these, the heart, with its four compartments, is 

 formed, and above, on the side of the head, the rudiments of the higher sense or- 

 gans appear — the nose, eye, and ear. These highly important organs originate in 

 the very simplest forms. The organ of smell appears quite in front of the head, in 

 the shape of two little pits, above the mouth opening. The organ of sight, also in 

 the form of a pit, comes next behind the organ of smell, toward which a considera- 

 ble vesicular outgrowth of the fore brain grows on both sides of the head. Farther 



